Let $ABC$ be a triangle with incenter $I$ and circumcircle $\Gamma$. Circles $\omega_{B}$ passing through $B$ and $\omega_{C}$ passing through $C$ are tangent at $I$. Let $\omega_{B}$ meet minor arc $AB$ of $\Gamma$ at $P$ and $AB$ at $M\neq B$, and let $\omega_{C}$ meet minor arc $AC$ of $\Gamma$ at $Q$ and $AC$ at $N\neq C$. Rays $PM$ and $QN$ meet at $X$. Let $Y$ be a point such that $YB$ is tangent to $\omega_{B}$ and $YC$ is tangent to $\omega_{C}$. Show that $A,X,Y$ are collinear.
Problem
Source: 2020 ISL G8
Tags: geometry, incenter, circumcircle, IMO Shortlist, IMO Shortlist 2020, tangent, Hi
20.07.2021 23:54
[asy][asy] /* Geogebra to Asymptote conversion, documentation at artofproblemsolving.com/Wiki go to User:Azjps/geogebra */ import graph; size(15cm); real labelscalefactor = 0.5; /* changes label-to-point distance */ pen dps = linewidth(0.7) + fontsize(10); defaultpen(dps); /* default pen style */ pen dotstyle = black; /* point style */ real xmin = -10.169, xmax = 8.2, ymin = -3, ymax = 7.3; /* image dimensions */ draw((-3.142967880809327,3.741704410289035)--(-4.27,-1.08)--(0.99,-1.08)--cycle, linewidth(1) + blue); /* draw figures */ draw((-3.142967880809327,3.741704410289035)--(-4.27,-1.08), linewidth(1) + blue); draw((-4.27,-1.08)--(0.99,-1.08), linewidth(1) + blue); draw((0.99,-1.08)--(-3.142967880809327,3.741704410289035), linewidth(1) + blue); draw(circle((-1.64,0.8478292787166926), 3.2608933941294866), linewidth(1)); draw(circle((-3.7362180134430876,0.22962892231873314), 1.4142316370900383), linewidth(1)); draw(circle((-0.16375861470797104,0.7966499870857753), 2.20294668910573), linewidth(1)); draw((-1.1069556946477261,-2.3692018831480532)--(-4.27,-1.08), linewidth(1)); draw((-1.1069556946477261,-2.3692018831480532)--(0.99,-1.08), linewidth(1)); draw(circle((-5.097700151769647,3.090432973082939), 2.060371989727628), linewidth(1)); draw(circle((-0.6303772705960715,4.578841302197107), 2.648378702211095), linewidth(1)); draw((-2.605842442388212,2.1295710231108576)--(-3.142967880809327,3.741704410289035), linewidth(1)); draw((-2.605842442388212,2.1295710231108567)--(xmax, -6.300399792383099*xmax-14.288278159894903), linewidth(1)); /* ray */ draw((-4.815305376023,1.5900297212366055)--(-1.64,-2.4130641154127943), linewidth(1)); /* dots and labels */ dot((-3.142967880809327,3.741704410289035),dotstyle); label("$A$", (-3.051170840703382,3.956205358613116), NE * labelscalefactor); dot((-4.27,-1.08),dotstyle); label("$B$", (-4.173819337640587,-0.8582295417137585), NE * labelscalefactor); dot((0.99,-1.08),dotstyle); label("$C$", (1.0724034461236613,-0.8582295417137585), N * labelscalefactor); dot((-1.64,-2.4130641154127943),linewidth(4pt) + dotstyle); label("$M_{A}$", (-1.5615026428443977,-2.2399507687133995), S *6* labelscalefactor); dot((-2.339470425047464,0.4513208203605414),linewidth(4pt) + dotstyle); label("$I$", (-2.2523632563442164,0.6314386561452294), NE * labelscalefactor); dot((0.8358349509755364,2.970013771970114),linewidth(4pt) + dotstyle); label("$M_{B}$", (0.921277686920576,3.135808380082079), N * labelscalefactor); dot((-4.815305376023,1.5900297212366055),linewidth(4pt) + dotstyle); label("$M_{C}$", (-4.73514358610919,1.7540871530824378), W * 2*labelscalefactor); dot((-2.605842442388212,2.1295710231108576),dotstyle); label("$X$", (-2.5114359864066484,2.3370007957229113), SW * 4*labelscalefactor); dot((-3.6341824169645403,1.640174866411884),linewidth(4pt) + dotstyle); label("$M$", (-3.5477269066563766,1.818855335598046), NW * labelscalefactor); dot((-1.841899916754811,2.2238205744391006),linewidth(4pt) + dotstyle); label("$N$", (-1.7558071903912218,2.4017689782385196), N * labelscalefactor); dot((-4.895223969272359,1.040033955434055),linewidth(4pt) + dotstyle); label("$P$", (-4.799911768624798,1.214352298785703), W * 2*labelscalefactor); dot((1.1171955142980874,2.5888917772536733),linewidth(4pt) + dotstyle); label("$Q$", (1.2019398111548774,2.7687886791602994), N * labelscalefactor); dot((-1.1069556946477261,-2.3692018831480532),linewidth(4pt) + dotstyle); label("$Y$", (-1.0217677885476644,-2.196771980369661), NE * labelscalefactor); clip((xmin,ymin)--(xmin,ymax)--(xmax,ymax)--(xmax,ymin)--cycle); /* end of picture */ [/asy][/asy] First note $Y \in (ABC)$ by a simple angle chase:$$\angle ABY + \angle ACY = (180^\circ - \angle MIB)+(180^\circ - \angle NIC) = \angle BIC + \angle MIN = (90^\circ + \frac{\angle A}{2})+\angle MBI + \angle NCI = 180^\circ.$$Key Claims: $MNPQ$ is cyclic and $(MPA),(NQA)$ are tangent at $A$. Proof: Invert about the incircle: $I$ is now the orthocenter of $\triangle ABC$, and $\overline{BPM} \parallel \overline{CQN}$. Since $$\angle APM = 180^\circ - \angle APB = \angle ACB = 180^\circ - \angle AIB = \angle AMP$$we have $AM=AP$, and similarly $AN=AQ$. The claim follows (since $MP \parallel NQ$). Return to the uninverted picture. Since $X$ is the radical center of the circles $\omega_B, \omega_C, (MPA), (NQA)$ we have $XA=XI$. Let $M_A$ be the midpoint of $\widehat{BC}$ not containing $A$, and define $M_B, M_C$ similarly. Then the desired result follows from Pascal on $AYBM_BM_CM_A$.
21.07.2021 00:02
21.07.2021 03:04
[asy][asy] size(6cm); real labelscalefactor = 0.5; /* changes label-to-point distance */ pen dps = linewidth(0.7) + fontsize(10); defaultpen(dps); /* default pen style */ pen dotstyle = black; /* point style */ real xmin = -4.68, xmax = 13.16, ymin = -3.5, ymax = 6.98; /* image dimensions */ pen zzttff = rgb(0.6,0.2,1); pen ffvvqq = rgb(1,0.3333333333333333,0); /* draw figures */ draw(circle((-1.0964533667921372,2.808556043496445), 3.1646044581100514), linewidth(0.8) + zzttff); draw(circle((4.863546633207864,2.4885560434964464), 3.164604458110053), linewidth(0.8) + zzttff); draw(circle((3.276453366792137,0.8314439565035552), 3.1646044581100523), linewidth(0.8) + ffvvqq); draw((-0.35870469944593764,5.885965389053015)--(0.24,-0.06), linewidth(0.8)); draw((0.24,-0.06)--(6.2,-0.38), linewidth(0.8)); draw((6.2,-0.38)--(5.601295300554062,5.565965389053019), linewidth(0.8)); draw((5.601295300554062,5.565965389053019)--(-0.35870469944593764,5.885965389053015), linewidth(0.8)); draw((1.94,3.7)--(1.827093266415726,1.5971120869928908), linewidth(0.8)); draw((0.24,-0.06)--(1.94,3.7), linewidth(0.8)); draw((1.94,3.7)--(6.2,-0.38), linewidth(0.8)); draw((0.24,-0.06)--(1.827093266415726,1.5971120869928908), linewidth(0.8)); draw((1.827093266415726,1.5971120869928908)--(6.2,-0.38), linewidth(0.8)); draw((-0.35870469944593764,5.885965389053015)--(1.827093266415726,1.5971120869928908), linewidth(0.8)); draw((1.827093266415726,1.5971120869928908)--(5.601295300554062,5.565965389053019), linewidth(0.8)); /* dots and labels */ dot((1.94,3.7),dotstyle); label("$A'$", (1.76,4.26), NE * labelscalefactor); dot((0.24,-0.06),dotstyle); label("$B'$", (-0.4,-0.48), NE * labelscalefactor); dot((6.2,-0.38),dotstyle); label("$C'$", (6.28,-0.18), NE * labelscalefactor); dot((1.827093266415726,1.5971120869928908),linewidth(4pt) + dotstyle); label("$I$", (1.5,1.54), NE * labelscalefactor); dot((-0.35870469944593764,5.885965389053015),dotstyle); label("$M'$", (-0.28,6.08), NE * labelscalefactor); dot((0.12323437020220891,1.0996441351657311),linewidth(4pt) + dotstyle); label("$P'$", (0.2,1.26), NE * labelscalefactor); dot((5.601295300554062,5.565965389053019),linewidth(4pt) + dotstyle); label("$N'$", (5.68,5.72), NE * labelscalefactor); dot((5.899799138628696,2.6014095882821118),linewidth(4pt) + dotstyle); label("$Q'$", (5.98,2.76), NE * labelscalefactor); clip((xmin,ymin)--(xmin,ymax)--(xmax,ymax)--(xmax,ymin)--cycle); [/asy][/asy] Claim. $M,N,P,Q$ are concyclic. Proof. Consider the inversion at $I$ which preserves $(ABC)$, let $X'$ be the image of an object $X$. Then $I$ is the orthocenter of $A'B'C'$. Let $A=\angle B'A'C'$, $B=\angle A'B'C'$ and $C=\angle A'C'B'$. Notice that $B'M'\|C'N'$. Hence $$\angle M'IB'+\angle N'IC'=360^{\circ}-\angle B'M'I-\angle M'B'I-\angle IN'C'-\angle IC'N'=360^{\circ}-A-(180^{\circ}-A)=180^{\circ}$$hence $$M'B'=\frac{B'I\sin\angle M'IB'}{\sin\angle B'M'I}=\frac{C'I\sin\angle C'IN'}{\sin\angle C'N'I}=N'C'$$which implies $M'N'C'B'$ is a parallelogram, hence $M'N'=BC=P'Q'$, which implies $M'N'Q'P'$ is an isoceles trapezoid, which is indeed cyclic, invert back yields the result. $\square$ [asy][asy] size(10cm); real labelscalefactor = 0.5; /* changes label-to-point distance */ pen dps = linewidth(0.7) + fontsize(10); defaultpen(dps); /* default pen style */ pen dotstyle = black; /* point style */ real xmin = -9.871875000000001, xmax = 15.215624999999994, ymin = -9.28312499999999, ymax = 5.454374999999993; /* image dimensions */ pen qqwuqq = rgb(0,0.39215686274509803,0); pen ffvvqq = rgb(1,0.3333333333333333,0); pen zzttff = rgb(0.6,0.2,1); pen fuqqzz = rgb(0.9568627450980393,0,0.6); pen zzttqq = rgb(0.6,0.2,0); /* draw figures */ draw(circle((-4.26,-1.6), 2.1883326986543885), linewidth(0.8) + qqwuqq); draw(circle((2.1093749999999964,-0.6768749999999999), 4.24758974154406), linewidth(0.8) + qqwuqq); draw((-5.38,-3.48)--(4.663783792716652,-4.070547690782389), linewidth(0.8)); draw((-5.38,-3.48)--(-2.09429466218645,-1.286120421553271), linewidth(0.8)); draw((-2.09429466218645,-1.286120421553271)--(4.663783792716652,-4.070547690782389), linewidth(0.8)); draw(circle((-0.23732540859718557,-1.7210531721695757), 5.435162913491629), linewidth(0.8) + blue); draw(circle((-6.5227070845296256,11.380965686923354), 11.108458046184419), linewidth(0.8) + linetype("4 4") + ffvvqq); draw((-3.1794087991084052,2.8489732193814428)--(-5.38,-3.48), linewidth(0.8)); draw((-3.1794087991084052,2.8489732193814428)--(4.663783792716652,-4.070547690782389), linewidth(0.8)); draw((-5.264927145060398,0.34394481226210705)--(-2.402847685913124,0.8428327705919746), linewidth(0.8) + zzttff); draw((-2.402847685913124,0.8428327705919746)--(1.2985163948385723,3.492600618696674), linewidth(0.8) + zzttff); draw((-5.264927145060398,0.34394481226210705)--(-1.7609218532430773,-6.938298591934416), linewidth(0.8) + linetype("4 4") + fuqqzz); draw((-1.7609218532430773,-6.938298591934416)--(1.2985163948385723,3.492600618696674), linewidth(0.8) + linetype("4 4") + fuqqzz); draw((-1.7609218532430773,-6.938298591934416)--(-3.1794087991084052,2.8489732193814428), linewidth(0.8)); draw((0.6643662859899618,-7.08089906399402)--(-3.1794087991084052,2.8489732193814428), linewidth(0.8) + linetype("4 4") + zzttqq); /* dots and labels */ dot((-5.38,-3.48),dotstyle); label("$B$", (-5.990625000000002,-3.995624999999996), NE * labelscalefactor); dot((-4.26,-1.6),dotstyle); label("$O$", (-4.134375000000002,-1.32375), NE * labelscalefactor); dot((2.1093749999999964,-0.6768749999999999),dotstyle); label("$O_{1}$", (2.2218749999999963,-0.3956250000000002), NE * labelscalefactor); dot((-2.09429466218645,-1.286120421553271),linewidth(4pt) + dotstyle); label("$I$", (-1.96875,-1.0706249999999995), NE * labelscalefactor); dot((4.663783792716652,-4.070547690782389),dotstyle); label("$C$", (4.78125,-3.79875), NE * labelscalefactor); dot((-3.1794087991084052,2.8489732193814428),linewidth(4pt) + dotstyle); label("$A$", (-3.0656250000000025,3.06375), NE * labelscalefactor); dot((-3.3680335904423444,-3.5982982570729587),linewidth(4pt) + dotstyle); label("$D$", (-3.2625,-3.3768749999999965), NE * labelscalefactor); dot((-0.825135705191316,-3.747813868422477),linewidth(4pt) + dotstyle); label("$E$", (-0.703125000000003,-3.5175), NE * labelscalefactor); dot((-5.264927145060398,0.34394481226210705),linewidth(4pt) + dotstyle); label("$P$", (-5.146875000000002,0.5606249999999986), NE * labelscalefactor); dot((-3.9720518999683647,0.5693053938272891),linewidth(4pt) + dotstyle); label("$M$", (-3.853125000000002,0.7856249999999984), NE * labelscalefactor); dot((-1.5762320381717025,1.4345982706453033),linewidth(4pt) + dotstyle); label("$N$", (-1.4625,1.6575), NE * labelscalefactor); dot((1.2985163948385723,3.492600618696674),linewidth(4pt) + dotstyle); label("$Q$", (1.40625,3.710624999999995), NE * labelscalefactor); dot((-2.402847685913124,0.8428327705919746),linewidth(4pt) + dotstyle); label("$X$", (-2.278125000000003,1.066874999999998), NE * labelscalefactor); dot((0.6643662859899618,-7.08089906399402),linewidth(4pt) + dotstyle); label("$Y$", (0.7875,-6.864374999999993), NE * labelscalefactor); dot((-1.7609218532430773,-6.938298591934416),linewidth(4pt) + dotstyle); label("$Z$", (-1.659375000000003,-6.72375), NE * labelscalefactor); clip((xmin,ymin)--(xmin,ymax)--(xmax,ymax)--(xmax,ymin)--cycle); [/asy][/asy] Now we return to the original problem, let $\omega_B$ and $\omega_C$ intersect $BC$ at $D,E$ respectively. Let the isogonal line of $AY$ w.r.t. $\angle BAC$ meet $(ABC)$ at $Z$. Firstly, we have $$\angle BYC=180^{\circ}-\angle YBC-\angle YCB=180^{\circ}-\angle BMD-\angle CNE=180^{\circ}-\angle BAC$$The last equality follows from the fact that $MD,NE$ are both perpendicular to the radical axis of $\omega_B$ and $\omega C$, so $Y$ lies on $(BAC)$. Notice that $$\angle BPD=\angle YBC=\angle BPZ$$hence $P,D,Z$ are collinear. Now by spiral sim. lemma $$\triangle PAM\sim\triangle PCD$$hence $$\angle APM=\angle CPD=\angle CPZ=\angle BAY$$so $AY$ is tangent to $(APM)$, and by symmetry tangent to $(AQN)$ as well. By radical axis on $(APM),(AQN)$ and $(MNPQ)$, $MN\cap PQ=X$ lies on $AY$ as desired.
21.07.2021 11:19
21.07.2021 12:37
Let \(\overline{PM}\) and \(\overline{QN}\) intersect \(\Gamma\) again at \(P'\) and \(Q'\). Claim: \(BCNM\) is tangential, and \(Y\in\Gamma\). Proof. If \(\ell\) is the common tangent to \(\omega_B\) and \(\omega_C\) at \(I\), then \begin{align*} \measuredangle MIN&=\measuredangle(\overline{MI},\ell)+\measuredangle(\ell,\overline{NI})\\ &=\measuredangle MBI+\measuredangle ICN=\measuredangle IBC+\measuredangle BCI=\measuredangle BIC, \end{align*}implying that \(BCNM\) is tangential. Moreover \[\measuredangle YBM=\measuredangle BIM=\measuredangle CIN=\measuredangle YCN,\]so \(Y\) lies on \(\Gamma\). \(\blacksquare\) [asy][asy] size(8cm); defaultpen(fontsize(10pt)); pen pri=blue; pen pri2=lightblue; pen sec=heavygreen; pen tri=orange; pen qua=lightred; pen fil=invisible; pen fil2=invisible; pen sfil=invisible; pen tfil=invisible; pen qfil=invisible; pair A,B,C,I,P,M,T,NN,Q,X,Pp,Qp,Y,D; A=dir(115); B=dir(225); C=dir(315); I=incenter(A,B,C); P=dir(200); M=2*foot(circumcenter(B,I,P),A,B)-B; T=reflect(M,I)*foot(I,A,B); NN=extension(A,C,M,T); Q=reflect(origin,circumcenter(C,I,NN))*C; X=extension(P,M,Q,NN); Pp=2*foot(origin,P,M)-P; Qp=2*foot(origin,Q,NN)-Q; Y=2*foot(origin,A,X)-A; D=foot(I,B,C); draw(circumcircle(M,P,Q),tri+linewidth(.3)+dashed); draw(circumcircle(A,M,P),qua+dashed); draw(circumcircle(A,NN,Q),qua+dashed); draw(A--Y,qua+linewidth(1)); draw(Pp--Qp,pri2+linewidth(1)); draw(P--Pp,tri+linewidth(.3)); draw(Q--Qp,tri+linewidth(.3)); filldraw(circumcircle(B,I,M),sfil,sec); filldraw(circumcircle(C,I,NN),sfil,sec); draw(M--NN,pri); filldraw(unitcircle,fil,pri); filldraw(incircle(A,B,C),fil2,pri2); filldraw(A--B--C--cycle,fil,pri); dot("\(A\)",A,NW); dot("\(B\)",B,SW); dot("\(C\)",C,SE); dot("\(M\)",M,NW); dot("\(N\)",NN,dir(80)); dot("\(I\)",I,E); dot("\(T\)",T,dir(75)); dot("\(P\)",P,dir(160)); dot("\(Q\)",Q,dir(-10)); dot("\(P'\)",Pp,Pp); dot("\(Q'\)",Qp,Qp); dot("\(X\)",X,NW); dot("\(Y\)",Y,S); dot("\(D\)",D,S); real t=.2; clip( (-1-t,-1-t)--(1+t,-1-t)--(1+t,1+t)--(-1-t,1+t)--cycle); [/asy][/asy] Claim: \(\overline{YA}\) is tangent to \((AMP)\) and \((ANQ)\). Proof. Note that \[\measuredangle APM=\measuredangle APB+\measuredangle BPM=\measuredangle AYB+\measuredangle YBM=\measuredangle YAM,\]and analogously \(\measuredangle AQN=\measuredangle YAN\). \(\blacksquare\) Claim: \(M\), \(N\), \(P\), \(Q\) are concyclic. Proof. If \(\overline{PM}\) and \(\overline{QN}\) intersect \(\Gamma\) again at \(P'\) and \(Q'\), then By Reim's theorem it will suffice to show \(\overline{MN}\parallel\overline{P'Q'}\). If the incircle touches \(\overline{BC}\) and \(\overline{MN}\) at \(D\) and \(T\), then \begin{align*} \measuredangle(\overline{P'Q'},\overline{BC}) &=\measuredangle Q'P'C+\measuredangle P'CB =\measuredangle NQC+\measuredangle MPB\\ &=\measuredangle NIC+\measuredangle MIB =2\measuredangle MIB=\measuredangle TID=\measuredangle(\overline{MN},\overline{BC}), \end{align*}as required. \(\blacksquare\) By radical axis theorem on \((AMP)\), \((ANQ)\), \((MNPQ)\), the desired conclusion follows, with points \(A\), \(P\), \(Q\) on the common tangent of \((AMP)\) and \((ANQ)\).
21.07.2021 13:03
EGMO 2019 P4 vibe
21.07.2021 16:18
Solution by L567 We will ignore $\omega_C$ for now. Let $R,S$ be midpoints of minor arcs $AC$ and $AB$. Define $X$ instead as $RS \cap PM$ and redefine $Y$ as tangent to $\omega_B$ meeting $(ABC)$. Let $M_A$ be the third arc midpoint First, see that $\angle XSI = \angle RSI = \angle RBC = \angle MBI = \angle MPI = \angle XPI$, which means $PXIS$ is cyclic. Next, since $\angle XIM + \angle MIP = \angle XIP = \angle XSP = \angle RSP = \angle RBP = \angle IBP = \angle IMX = \angle MPI + \angle MIP$, we have $\angle XIM = \angle MPI$, which means $XI$ is tangent to $\omega_B$ Now, we have $\angle YAI = \angle YAM_A = \angle YBM_A = \angle IBM_A - \angle IBY = \angle IBM_A - \angle IMB$ Let $AI \cap \omega_B = Z$ Since $X$ lies on $RS$, the perpendicular bisector of $AI$, $\angle XAI = \angle XIZ = \angle ZMI = \angle BMZ - \angle IMB$ But since $\angle BMZ = 180 - \angle AIB = \angle IBM_A$, this means $\angle YAI = \angle XAI \implies A,X,Y$ are collinear. Since we have $\angle BYI = 360 - (\angle IBY - \angle ICY) - \angle BIC = 360 - 2 \angle BIC = 180 - \angle BAC$ (The middle thing follows because the circles are tangent) which means $Y \in (ABC)$ and so $Y$ is indeed the $Y$ in the problem. So, $PM$ and $QN$ both meet $RS$ at a point on $AY$. So, $X = PM \cap QN$. So, $X$ is also the $X$ in the problem. So, we indeed have $A,X,Y$ collinear, as desired. $\blacksquare$
Attachments:

22.07.2021 03:11
22.07.2021 07:41
Easy for a G8? Here is a solution using only angle chasing. Let $E,F$ be the intersections of $\omega_B,\omega_C$ such that $E \neq B, F \neq C$. Let $U = \overline{EM} \cap \overline{AC}$ and $V = \overline{FN} \cap \overline{AB}$. Claim 1: $(MNEF)$ is an iscosceles trapezoid and $Y \in \odot(ABC)$ Proof: Since $\overline{BI}$ is an angle bisector so $\overline{IM} = \overline{IE}$. Similiarly $\overline{IN} = \overline{IF}$ . Thus $\overline{O_1O_2}$ is a perpendicular bisector of $\overline{ME},\overline{NF}$ implying the first result. Now we have $$\angle YBE = \angle BME = \angle BAC - \angle FNC = \angle BAC - \angle FCY$$implying the second part $\qquad \square$ Claim 2: $\overline{YA}$ tangent to $\odot(AMP),\odot(ANQ)$ Proof: Notice that $\angle MPE = \angle ABC$ and $\angle APB = 180^\circ - \angle ACB$ and so $$\angle APM = \angle BAC - \angle BPE = \angle BAC - \angle YBE = \angle FCY = \angle BAY$$ Implying $\overline{YA}$ tangent to $\odot(AMP)$. Similiarly we can prove the other part $\qquad \square$ Claim 3: $U \in \odot(AMP), V \in \odot(ANQ)$ Proof: $$\angle AUM = \angle CNF = \angle BCY = \angle MAY = \angle MPA$$similiarly the other part $\qquad \square$ Claim 4: $(MNQP)$ cyclic Proof: Notice that $$\angle QPM = \angle APM - \angle APQ = \angle FNC - \angle APQ = \angle FNC - \angle NCQ$$also note that $$\angle MNQ = \angle MNV + \angle VNQ = \angle NFC + \angle QCF$$and so $$\angle QPM + \angle MNQ = \angle FNC + \angle NFC + \angle QCF - \angle NCQ = 180^\circ$$hence the result $\qquad \square$ Finish by applying radical axis on $\odot(MNQP), \omega_B,\omega_C$ $\qquad \blacksquare$
23.07.2021 10:59
Let $\triangle DEF$ be the contact triangle of $\triangle ABC$. Invert about $(DEF)$. Let the image of $\bullet$ after the inversion be $\bullet'$. Then $A',B',C'$ are the midpoints of $EF,FD,DE$ respectively. Note that $I$ is the orthocenter of $\triangle A'B'C'$. Also, $(BPMI)$ and $(CQNI)$ map to two parallel lines, $\overline{B'P'M'}$ and $\overline{C'Q'N'}$, where $P'$ and $Q'$ lie on $(A'B'C')$ and $M'$ and $N'$ lie on $(A'IB'F)$ and $(A'IC'E)$, respectively. Now $X'=(M'P'I) \cap (N'Q'I)$. Finally, $Y'$ lies on the intersection of the two circles through $I$ tangent to $\overline{B'P'M'}$ and $\overline{C'Q'N'}$ at $B'$ and $C'$, respectively. From here on out, we omit the $'$ in $\bullet '$. Note that the circumradii of $\triangle ABC, \triangle AIBF,$ and $\triangle AICE$ are all equal to half the circumradius of $\triangle DEF$. Then $$\measuredangle APM = \measuredangle APB = \measuredangle BIA = \measuredangle BMA = \measuredangle PMA,$$so $AP=AM$. Similarly, $AQ=AN$. Thus, $PMNQ$ is an isosceles trapezoid. We delete points $D,E,$ and $F$, and restate the problem, and relabel $I$ as $H$. New Problem wrote: Let $\triangle ABC$ have circumcircle $\Gamma$. Two parallel lines, $\ell_B$ and $\ell_C$, pass through $B$ and $C$, respectively. Let $\ell$ be a line through $A$ perpendicular to $\ell_B$ and $\ell_C$. Let $P = \ell_B \cap \Gamma \neq B$ and $Q = \ell_C \cap \Gamma \neq C$, and let $M$ and $N$ be the reflections of $P$ and $Q$ over $\ell$, respectively. Let $X = (PMH) \cap (QNH) \neq H$, and let $Y$ be the intersection of the two circles through $H$ and tangent to $\ell_B$ and $\ell_C$ at $B$ and $C$, respectively. Prove that $A,X,H,$ and $Y$ lie on a circle. The first observation we make is that by symmetry, $X$ is just the reflection of $H$ over $\ell$. We can now delete $M,P,N,$ and $Q$. [asy][asy] defaultpen(fontsize(12)); size(12cm); pair A,B,C,H,Y,OB,X,TB,TC; A = dir(110); B = dir(210); C = dir(330); H = orthocenter(A,B,C); Y = dir(260); OB = circumcenter(B,H,Y); X = 2*foot(H,A,A+B-OB)-H; TB = B + (B-OB)*dir(90); TC = C + (B-OB)*dir(90); draw(circumcircle(A,B,C),red); filldraw(A--B--C--cycle,blue+white+white+white); draw(circumcircle(B,Y,H),green); draw(circumcircle(C,Y,H),green); draw(TB--B+(B-TB)*5,magenta); draw(TC--C+(C-TC)*5,magenta); draw(A+2*(B-OB)--A-4*(B-OB),blue); draw(X--H,dashed); label("$\ell_B$",B+(B-TB)*5,dir(0)); label("$\ell_C$",C+(C-TC)*5,dir(0)); label("$\ell$",A-4*(B-OB),dir(270)); dot("$A$",A,dir(A)); dot("$B$",B,dir(180)); dot("$C$",C,dir(350)); dot("$Y$",Y,dir(270)); dot("$X$",X,dir(0)); dot("$H$",H,dir(300)*1.5); real r = circumradius(X,A,H); draw(arc(circumcenter(X,A,H),circumcenter(X,A,H)+r*dir(160),circumcenter(X,A,H)+r*dir(200)),dotted+purple); [/asy][/asy] Since $AH=AX$, we prove instead that the tangent at $A$ to $(AHY)$ is parallel to $\ell_B$ and $\ell_C$. We now finish the problem with the following lemma: Lemma: If $\triangle ABC$ is a triangle, $P$ is a point on $(ABC)$, and $H$ is the orthocenter of $\triangle ABC$, then the tangent to $(BPH)$ at $B$ and the tangent to $(CPH)$ at $C$ are parallel if and only if $P$ lies on $(ABC)$. Proof: Suppose the two tangent lines are parallel, and call them $\ell_B$ and $\ell_C$. Then $$\measuredangle BPC = \measuredangle BPH + \measuredangle HPC = \measuredangle (\ell_B, BH) + \measuredangle (CH, \ell_C) = \measuredangle (\ell_B,BC) + \measuredangle CBH + \measuredangle(BC,\ell_C) + \measuredangle HCB$$$$= \measuredangle CBH + \measuredangle HCB = \measuredangle CHB = \measuredangle BAC,$$so $P$ lies on $\Gamma$. If $P$ lies on $\Gamma$, then using the above equation, $\measuredangle (\ell_B, BC) + \measuredangle (BC, \ell_C) = 0^\circ$, so $\ell_B \parallel \ell_C$. $\blacksquare$ Now, using this lemma, $Y$ lies on $\Gamma$, and again using this lemma, the tangent to $(AHY)$ at $A$ is parallel to $\ell_B$ and $\ell_C$, as desired.
23.07.2021 12:44
Claim-1-$Y \in (ABC)$ Proof: Let $l$ be the common tangent of $w_b$ and $w_c$ If $l \cap BY=J$ and $l \cap CY=J_1$ then by tangent secant theorem $$\angle YBI= \angle JIB=\angle BMI \; \; \; \; \angle YCI= \angle J_1IC=\angle CNI$$$$\implies \angle YBI +\angle YCI=\angle BIC$$But notice that if $CI \cap (ABC)=H$ and $BI \cap (ABC)=K$ then $\angle HBI=\frac{B}{2}+\frac{C}{2}=90-\frac{A}{2}$ If we take the angles to be directed it is clear that $Y \in (ABC)$ Claim-2- $X = HK \cap l$ Proof: Assume a fantom point $Y= l \cap HK$ Now, because of symmetry it suffice to show that $Y \in PM$ Notice that $\angle YIP=\angle PBI=\angle PHK \implies Y \in (HPI)$ Further $\angle YPI=\angle YHI=\frac{B}{2}=\angle MBI=\angle MPI \implies P-M-X$ To finish, observe that $HK$ is the perpendicular bisector of $AI$ (well known, also trivial using angle chasing). Hence $\angle KAX=\angle KIX=\angle IBY$ Hence we are done!
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23.07.2021 18:44
Let $\omega_P$ and $\omega_Q$ denote the circumcircles of $\triangle APM$ and $\triangle AQN$. By the same trick used in G3 and G6, \[ \measuredangle MIN = \measuredangle MBI + \measuredangle ICN = \measuredangle IBC + \measuredangle BCI = \measuredangle BIC, \]so $I$ has an isogonal conjugate with respect to $BCNM$. Hence it must also be the incenter of $BCNM$. Claim 1: $\omega_P$ and $\omega_Q$ are tangent to each other. Proof. This is just angle chasing. \begin{align*} \measuredangle APM + \measuredangle NQA &= \measuredangle APB + \measuredangle BPM + \measuredangle NQC + \measuredangle CQA \\ &= \measuredangle ACB + \measuredangle BIM + \measuredangle NIC + \measuredangle CBA\\ &= \measuredangle CAB. \qquad\square \end{align*} Let their common tangent at $A$ be $\ell$. Claim 2: $P$, $Q$, $M$, $N$ are concyclic. By angle chasing again, one has \begin{align*} \measuredangle QPM &= \measuredangle QPB + \measuredangle BPM\\ &= \measuredangle QCB + \measuredangle BIM\\ &= \measuredangle QCI + \measuredangle ICB + \measuredangle CIN\\ &= \measuredangle QNI + \measuredangle NCI + \measuredangle CIN\\ &= \measuredangle QNI + \measuredangle CNI\\ &= \measuredangle QNI + \measuredangle INM = \measuredangle QNM.\qquad\square \end{align*} Claim 3: $Y$ lies on $(ABC)$. Proof. Let $M_A$, $M_B$ and $M_C$ be midpoints of arcs $BC$, $CA$ and $AB$. Let $O_B$ and $O_C$ be the centers of $\omega_B$ and $\omega_C$. Since $O_B$, $I$, $C$ are collinear, by Pascal’s theorem $\overline{BO_B}$ and $\overline{CO_C}$ intersect on $(ABC)$. Thus $Y$ also lies on $(ABC)$. $\square$ By radical axes on $(PQNM)$, $\omega_P$, and $\omega_Q$, it follows that $X$ lies on $\ell$. Therefore, it suffices to show that $Y$ also lies on $\ell$. Just notice that \[ \measuredangle APM = \measuredangle APB + \measuredangle BPM = \measuredangle ACB + \measuredangle YBA = \measuredangle ACB + \measuredangle YCA = \measuredangle YCB = \measuredangle YAM, \]so $Y$ lies on $\ell$ as desired. $\blacksquare$
24.07.2021 01:46
This is really a cute problem imo so here goes the solution Claim 1:- $Y$ lies on $\odot(ABC)$ This can be proved by clever angle chase,Join $MI$ and $NI$ as $YB$ and $YC$ are tangent $\angle YBA$=$\angle BPM$=180-$\angle BIM$ $\angle YCN$=$\angle NQC$=180-$\angle NIC$ Now $\angle BIC$=90+$\frac{A}{2}$,$\angle XIN$=$\frac{C}{2}$, $\angle MIX$=$\frac{B}{2}$ By some computation one can easily get $\angle A$+$\angle BCY$=180$^\circ$ This concludes our claim. Claim 2:-M,N,P,Q are concyclic This is also simple angle chase with the use of dangles $\angle QPM$=$\angle QPB$-$\angle BPM$ =$\angle QCB$+$\angle BIM$-180 =$\angle QCI$+$\angle ICB$-$\angle CIN$ =180-$\angle QNI$+$\angle NCI$-$\angle CIN$ =-$\angle QNI$ - $\angle QCI$ =-$\angle QNM$ This concludes our claim and hence M,N,P,Q are concyclic Claim 3:- $YA$ is the radical axis $\odot (AMP)$ and $\odot (ANQ)$ This is again angle chase, BYPA is Concyclic so $\angle BYA$=180-$\angle BPA$ We know that $\angle ABY$=$\angle BPI$ From here, $\angle APM$=$\angle MAP$ similarly do the same angle chasing to prove $\angle XAN$=$\angle AQN$ and this concludes our claim. Claim 4:- $XA$ is the radical axis of $\odot (ANQ) $ and $\odot (AMP)$ As $XM \cdot XP=XN \cdot XQ$ it follows that X lies on the radical axis of $\odot (AMP)$ and $\odot (ANQ)$ Applying radical axis lemma we hence showed that $X-Y-A$ are collinear as desired.
27.07.2021 10:37
[asy][asy] size(11cm); defaultpen(fontsize(10pt)); pair A = dir(110), B = dir(220), C = dir(320), I = incenter(A,B,C), Y = dir(285), P1 = intersectionpoint(Y+dir(B--A)*0.001--Y+dir(B--A)*100, unitcircle), Q1 = intersectionpoint(Y+dir(C--A)*0.001--Y+dir(C--A)*100, unitcircle), X = extension(A,Y,circumcenter(A,I,B),circumcenter(A,I,C)), M = extension(P1,X,A,B), N = extension(Q1,X,A,C), P = X+dir(P1--X)*abs(A-X)*abs(X-Y)/abs(P1-X), Q = X+dir(Q1--X)*abs(A-X)*abs(X-Y)/abs(Q1-X), X1 = 2*foot(X,A,I)-X, M1 = extension(M,M+dir(Q1--B),B,C), N1 = extension(N,N+dir(P1--C),B,C); draw(A--B--C--A^^unitcircle, heavycyan); draw(circumcircle(B,I,M)^^circumcircle(C,I,N), heavygreen); draw(B--Q1^^M--M1^^N--N1^^C--P1, magenta); draw(P--P1^^Q--Q1, heavygreen); draw(arc(circumcenter(M,N,P),circumradius(M,N,P),60,120), heavygreen+dashed); draw(B--Y--C, heavygreen); draw(X--I^^A--X1, magenta); draw(X--X1^^A--I, magenta+dotted); draw(A--Y, orange+dashed); draw(extension(M,N,B,Q1)--extension(M,N,C,P1)^^Q1--P1, heavycyan); dot("$A$", A, dir(110)); dot("$B$", B, dir(225)); dot("$C$", C, dir(315)); dot("$Y$", Y, dir(285)); dot("$P_1$", P1, dir(45)); dot("$Q_1$", Q1, dir(135)); dot("$P$", P, dir(135)); dot("$Q$", Q, dir(30)); dot("$X$", X, dir(60)); dot("$M$", M, dir(120)); dot("$N$", N, dir(80)); dot("$M_1$", M1, dir(280)); dot("$N_1$", N1, dir(230)); dot("$I$", I, dir(180)); dot("$X'$", X1, dir(170)); [/asy][/asy] Define $M_1 = \omega_B \cap \overline{BC}$, $N_1 = \omega_C \cap \overline{BC}$, $P_1 = \overline{PM} \cap \Gamma$, and $Q_1 = \overline{QN} \cap \Gamma$. Note that $MNN_1M_1$ is an isosceles trapezoid because $\overline{MM_1}$ and $\overline{NN_1}$ share the same perpendicular bisector. By Reim's we have $\overline{MM_1} \parallel \overline{CP_1}$, so in fact $\overline{BQ_1} \parallel \overline{MM_1} \parallel \overline{II} \parallel \overline{NN_1} \parallel \overline{CP_1}$. It follows by symmetry that $\overline{MN} \parallel \overline{P_1Q_1}$, so $PMNQ$ is cyclic by Reim's. Then $XM \cdot XP = XN \cdot XQ$, so $X$ lies on the radical axis of $\omega_B$ and $\omega_C$, which is the common tangent at $I$. From $\angle BCY = \angle N_1NC = \angle ACP_1 = \angle AQ_1P_1$ (similarly $\angle CBY = \angle AP_1Q_1$) and $BC = P_1Q_1$, we have $Y \in \Gamma$ with $\triangle BCY \cong \triangle P_1Q_1A$. I now claim that $XA = XI$. Let $X'$ be the reflection of $X$ over $\overline{AI}$. We have $\angle BAX' = \angle CAY = \angle ABQ_1$, so $\overline{AX'} \parallel \overline{BQ_1} \parallel \overline{XI}$. But by construction $AX'IX$ is a kite, so it must be a rhombus. To finish, note that from $$XA^2 = XI^2 = XN \cdot XQ,$$$\overline{XA}$ must be tangent to $(ANQ)$. Therefore $$\angle CAX = \angle NAX = \angle AQN = \angle AQQ_1 = \angle CAY$$which implies $A$, $X$, $Y$ are collinear, as required. $\blacksquare$
04.08.2021 19:39
Didn't want to use directed angles since I'm not much used to them but was forced to use it in some places. Also extremely long proof because I wrote each and every small detail (which are equally important) as claims :< Claim: $Y$ lies on $\Gamma$. Proof: Let $\ell$ be the common tangent of $\omega_B , \omega_C$, $G=\omega_B \cap BC$ and $H=\omega_C \cap BC$. \begin{align*} \angle YBA + \angle YCA &= \angle YBC + \frac{\angle B}{2} + \frac{\angle B}{2}+ \angle YCB + \frac{\angle C}{2} + \frac{\angle C}{2}\\ &= \angle BIG + \angle(MI , \ell ) + \angle (\ell , GI) + \angle CIH + \angle (\ell , NI) + \angle(HI , \ell)\\ &= \angle(MI , \ell ) + \angle (\ell , NI) + \angle BIG + \angle (\ell , GI) + \angle(HI , \ell) + \angle CIH\\ &= \angle MIN + \angle BIC \\ &= \frac{\angle B + \angle C}{2} + 90^\circ + \frac{\angle A}{2} \\ &=180^\circ \end{align*} Claim: $\angle IMN = \angle IMB$ and $\angle INM = \angle INC$. Proof: It's enough to show that $MI$ and $NI$ are angle bisectors of angle $MBN$ and $CNM$ respectively. But this is true because $I$ is midpoint of arc smaller $MG$ and smaller arc $NH$ so $IM = IG$ and $IN = IH$ and $$\angle MIN = \angle GIH = \dfrac{\angle B + \angle C}{2} \implies \triangle GIH \cong \triangle MIN$$So $\angle IMN = \angle IGH = \angle IMB$ and $\angle INM = \angle IHG = \angle INC$. Claim: $QPMN$ is cyclic. [Let's call the circumcircle $\omega$] Proof: (Sorry if I mess up dangles, it was really hard to write "$\pm 180^\circ , \pm 360^\circ$" in vanilla so many times ) \begin{align*} \measuredangle QPM &= \measuredangle QPB - \measuredangle MPB \\ &= \measuredangle QCH - \measuredangle MIB \\ &= \measuredangle QCI + \measuredangle ICH - \measuredangle MIB \\ &= \measuredangle QNI + \measuredangle NCI - \measuredangle NIC \\ &= \measuredangle QNI - \measuredangle INC \\ &= \measuredangle QNI + \measuredangle CNI \\ &= \measuredangle QNI + \measuredangle INM \\ &= \measuredangle QNM \end{align*}Now by Radical Axis theorem on $(\omega_B,\omega,\omega_C)$ we see that $PM$ and $QN$ concur on $\ell$ so $X \in \ell$. Claim: $XA=XI$. Proof: Let the perpendicular bisector of $AI$ intersect minor arc $AB$, $AC$ at $M_C$ and $M_B$ respectively. By Fact 5 $M_C$ and $M_B$ are the midpoints of the respective arcs. $$\measuredangle XIP = \measuredangle IBP = \measuredangle M_BBP = \measuredangle M_CM_BP = \measuredangle XM_CP$$So $XM_CPI$ is cyclic, similarly $XIM_BQ$ is cyclic. $$\measuredangle XM_CI = \measuredangle XPI = \measuredangle MPI = \measuredangle IBC = \measuredangle M_BBC = \measuredangle M_BM_CC = \measuredangle M_BM_CI$$So $X \in M_BM_C \implies X \in$ perpendicular bisector of $AI$. $XM \cdot XP = XI^2 = XA^2 \implies (MAP)$ is tangent to $AX$ thus $$\angle MPA = \angle MAX$$Let $F = PM \cap (ABC)$ Claim: $FABY$ is an isosceles trapezium. $$\angle FAB = \angle FPB = \angle MPB = \angle MBY = \angle YBA$$Finally to finish off $$\angle MPA = \angle FPA = \angle FYA = \angle BAY = \angle MAY$$$\implies \angle MAX = \angle MAY \implies \overline{A-X-Y}$ are collinear.
09.08.2021 05:39
[asy][asy] size(350); //stole geogebra points because i am too lazy to do asymptote correctly pair A=(2.999429319069,8.7145022896488), B = origin, C = (11.707076895692, 0); pair I = incenter(A, B, C); pair P = (-0.6582634654393, 3.0392879500232), Q = (5.4004931584466, 9.3572483063131), M = (1.6101594341007, 4.6781359327132), NN = (4.2178365368594, 7.4951359343011); pair X = extension(P, M, Q, NN); pair Y = (3.4938614778841, -3.2148956397776); draw(A--B--C--cycle, red); draw(circumcircle(A, B, C), blue); draw(circumcircle(B, P, I), orange); draw(circumcircle(C, Q, I), orange); draw(circumcircle(A, M, P), magenta); draw(circumcircle(A, NN, Q), magenta); pair OB = circumcenter(B, P, I), OC = circumcenter(C, Q, I); pair B1 = (3.6565170629108, 0), C1 = (7.4924761733508, 0); draw(M--B1, purple+dashed); draw(NN--C1, purple+dashed); draw(OB--OC, gray+dashed); pair Z = (8.2164182887114, 8.9293807829784); draw(B--Z--C--Y--cycle, green); draw(X--P, brown); draw(X--NN, brown); draw(X--Q, brown); draw(X--M, brown); draw(X--A, brown); draw(X--Y, brown); pair MB = (-0.3061221855607, 4.9787989737885), MC = (10.4616597135134, 7.4632127191535); draw(MB--MC, cyan); draw(A--I, gray+dashed); dot("$A$", A, N); dot("$B$", B, SW); dot("$C$", C, SE); dot("$I$", I, NE); dot("$O_B$", OB, S); dot("$O_C$", OC, NE); dot("$Y$", Y, S); dot("$X$", X, NW); dot("$M$", M, NW); dot("$N$", NN, NE); dot("$B_1$", B1, S); dot("$C_1$", C1, S); dot("$Z$", Z, NE); dot("$P$", P, W); dot("$Q$", Q, NE); dot("$M_C$", MB, W); dot("$M_B$", MC, NE); [/asy][/asy] Neat problem! Denote by $O_B$, $O_C$ the centers of $\omega_B$ and $\omega_C$, and let $\omega_B$ intersect $\overline{BC}$ at $B_1$, while $\omega_C$ intersects $\overline{BC}$ at $C_1$. We begin with the following claim. Claim. $\overline{MB_1}$ and $\overline{NC_1}$ are parallel. Proof. Observe that because $\overline{BI}$ and $\overline{CI}$ are angle bisectors, $I$ is the arc midpoint of $\widehat{MB_1}$ and $\widehat{NC_1}$, and hence $\overline{O_BO_C}$ is perpendicular to both $\overline{MB_1}$ and $\overline{NC_1}$. It follows that $\overline{MB_1}$ is parallel to $\overline{NC_1}$, as desired. $\blacksquare$ In fact we realize the stronger fact that $MNC_1B_1$ is an isosceles trapezoid, because the perpendicular bisectors of $\overline{MB_1}$ and $\overline{NC_1}$ coincide. Claim. $Y$ lies on the circumcircle $\Gamma$. Proof. Let $Z = \overline{BO_B} \cap \overline{CO_C}$. It suffices to show that $Z$ lies on $\Gamma$ by definition of $Y$. Observe that \begin{align*} \measuredangle BZC &= \measuredangle BO_BI + \measuredangle IO_CC \\ &= 2\measuredangle BMB_1 + B + 2\measuredangle C_1NC + C \\ &= 2(\measuredangle BB_1M + \measuredangle NC_1C) - B - C \\ &= \measuredangle BAC, \end{align*}because $\overline{MB_1} \parallel \overline{NC_1}$ by the first claim. This proves the result. $\blacksquare$ Claim. The triangles $XMN$ and $XQP$ are similar. Proof. We will show that $\measuredangle XMN = \measuredangle PQX$. Observe that \begin{align*} \measuredangle XMN &= \measuredangle B_1MN + \measuredangle XMB_1 \\ &= \measuredangle C_1B_1M + \measuredangle PBB_1 \\ &= \measuredangle B_1C_1N + \measuredangle PBB_1 \\ &=\measuredangle CQN + \measuredangle PQC \\ &= \measuredangle PQX, \end{align*}as required. This proves the claim. $\blacksquare$ From this claim, we obtain that $XM \cdot XP = XN \cdot XQ$, so $X$ lies on the radical axis of $(AMP)$ and $(ANQ)$. However, $X$ also lies on $\overline{PM}$ and $\overline{QN}$, so it has equal power to all of $\omega_B, \omega_C$, $(ANQ)$ and $(AMP)$. It follows that $X$ is the radical center of these four circles. Now, we prove the following: Claim. $\overline{XA}$ is the common tangent to $(AMP)$ and $(ANQ)$. Proof. First observe that $\overline{XI}$ is the common tangent of $\omega_B$ and $\omega_C$ because $X$ is the radical center. Hence it suffices to show that $XA = XI$. We will show that $X$ lies on the perpendicular bisector of $\overline{AI}$ -- in other words, $M_B, X, M_C$ are collinear where $M_B, M_C$ are the arc midpoints of minor $\widehat{AB}$ and $\widehat{AC}$. This is just angle chasing. First, we claim that $M_BXIP$ is cyclic. Indeed, this is just true by angle chasing: $$\measuredangle PXI = \measuredangle PMB_1 = \measuredangle PBC = \measuredangle PM_CC,$$in particular because $\overline{XI} \parallel \overline{MB_1}$, and $C, I, M_C$ are collinear. Then, we have \begin{align*} \measuredangle IXM_C &= \measuredangle IPM_C \\ &= \measuredangle BPM_C + \measuredangle IPB \\ &= \measuredangle BCI + \measuredangle IB_1C_1. \end{align*}For the next step, observe that $$\measuredangle IBC + \measuredangle IB_1C_1 = \measuredangle IPM + \measuredangle IPB = \measuredangle BB_1M = \measuredangle CC_1N = \measuredangle ICB + \measuredangle IC_1B_1,$$so this implies that $$\measuredangle IB_1C_1 + \measuredangle BCI = \measuredangle IC_1B_1 + \measuredangle CBI.$$Now we can just angle chase symmetrically in reverse around $C$, obtaining $$\measuredangle IXM_C = \measuredangle IXM_B,$$as desired. This proves the claim. $\blacksquare$ With this information, we can now find \begin{align*} \measuredangle XAB &= \measuredangle XAM \\ &= \measuredangle MPA \\ &= \measuredangle BPA + \measuredangle MPB \\ &= \measuredangle BCA + \measuredangle MB_1B \\ &= \measuredangle BCA + \measuredangle CC_1N \\ &= \measuredangle CNC_1 \\ &= \measuredangle YCB = \measuredangle YAB. \end{align*}Finally, this implies that $X, A, B$ are collinear, so we are done. $\square$
02.09.2021 07:54
Let $\omega_b, \omega_c$ meet $BC$ again at $D, E$ respectively, $M_b, M_c$ denote the midpoints of arcs $CA, AB$ respectively, $K$ be the second intersection between $PM$ and $\Gamma$, $T$ be the second intersection between $QN$ and $\Gamma$, and $l$ be the tangent at $I$. Claim: $Y \in (ABC)$. Proof. Notice $$180^{\circ} - \angle BYC = \angle YBC + \angle YCB = \angle BID + \angle CIE$$$$= \angle BIC - \angle DIE = \left(90^{\circ} + \frac{\angle A}{2} \right) - (\angle DBI + \angle ECI)$$$$= \left(90^{\circ} + \frac{\angle A}{2} \right) - \left(\frac{\angle B}{2} + \frac{\angle C}{2} \right) = \angle A$$which suffices. $\square$ Claim: $ABY \sim NEC$ and $ACY \sim MDB$. Proof. Observe $$\angle AYB = \angle ACB = \angle NCE$$and $$\angle BAY = \angle BCY = \angle ECY = \angle ENC$$implying the first similarity. The second result follows analogously. $\square$ Claim: $AB \parallel KY$ and $AC \parallel TY$. Proof. Because $P$ is the Miquel Point of $ACDM$, it's also the center of the spiral similarity taking $DM$ to $CA$. But the previous claim implies $B$ goes to $Y$ under the same transformation, so $PBDM \sim PYCA$. Now, the Spiral Similarity Lemma yields $PBY \sim PMA$. Hence, $$\angle ABK = \angle APK = \angle APM = \angle YPB = \angle YKB$$so $AB \parallel KY$. The second result follows in an analogous fashion. $\square$ Claim: $BT \parallel DM \parallel EN \parallel CK$. Proof. Notice $$\angle DMB = \angle CAY = \angle TYA = \angle TBA = \angle TBM$$so $BT \parallel DM$. Similarly, we conclude $CK \parallel EN$. Now, observe $$\angle BTC = \angle BTY + \angle YTC = \angle BAY + \angle ACT = \angle KYA + \angle AYT$$$$= \angle KYT = \angle KCT$$so $BT \parallel KC$. At this point, Transitivity of Parallelism finishes. $\square$ Claim: $MN \parallel KT$. Proof. Let $F = DM \cap KT$. Since $\angle DBI = \frac{\angle B}{2} = \angle MBI$, $I$ is the midpoint of arc $DM$, so $ID = IM$ and $DM \parallel l$. Similarly, we conclude $IE = IN$ and $EN \parallel l$, so $DENM$ is an isosceles trapezoid. As a result, $$\angle DMN = \angle MDE = \angle TBE = \angle TBC = \angle BTK = \angle DFK$$as desired. $\square$ Now, notice $$180^{\circ} - \angle PMN = \angle XMN = \angle XKT = \angle PKT = \angle PQT = \angle PQN$$so $MNQP$ is cyclic. Thus, $$Pow_{\omega_b}(X) = XM \cdot XP = Pow_{(MNQP)}(X) = XN \cdot XQ = Pow_{\omega_c}(X)$$so $X$ lies on the Radical Axis of $\omega_b$ and $\omega_c$, i.e. $l$. Claim: $XIPM_c$ is cyclic. Proof. Observe $$\angle IXP = 180^{\circ} - \angle XPI - \angle PIX = 180^{\circ} - \angle MPI - \angle PBI$$$$= 180^{\circ} - \angle MBI - \angle PBM_b = 180^{\circ} - \angle CBM_b - \angle PBM_b$$$$= 180^{\circ} - \angle CBP = \angle CM_cP = \angle IM_cP$$as required. $\square$ Claim: $X \in M_bM_c$. Proof. Notice $$\angle IM_cX = \angle IPX = \angle IPM = \angle IBM = \angle CBM_b = \angle CM_cM_b = \angle IM_cM_b$$which suffices. $\square$ It's well-known that $M_bM_c$ is the perpendicular bisector of $AI$, so $XA = XI$, which implies $$XA^2 = XI^2 = Pow_{\omega_b}(X) = XM \cdot XP.$$As a result, we know $XMA \sim XAP$, so $$\angle BAX = \angle MAX = \angle APX = \angle APK = \angle ABK = \angle YKB = \angle BAY$$which finishes the problem. $\blacksquare$
Attachments:

12.09.2021 06:13
We have $90^{\circ}+\frac{\angle BAC}{2}=\angle BIC = \angle BMI + \angle CNI = \angle IBY + \angle ICY = \frac{\angle ABC}{2} + \frac{\angle ACB}{2} + 180^{\circ} - \angle BAC$, which implies that $\angle BAC + \angle BAC = 180^{\circ}$ and $Y$ lies on $\Gamma$. We also have $\angle PAY = 180^{\circ}-\angle PBY = \angle PMB$, so $AY$ is tangent to $(AMP)$. Similarly, $AY$ is tangent to $(ANQ)$ and hence $(AMP)$ is tangent to $(ANQ)$. Now invert about $A$, we have $AY \parallel MP \parallel NQ$, so we need to prove that $AX \parallel MP \parallel NQ$. But this is easy. Notice that $I$ is now the midpoint of both the major arc $MP$ of $BMP$ and the major arc $NQ$ of $CNQ$, so $MP$ and $NQ$ share a common perpendicular bisector. This is also the line connecting the centers of $(AMP)$ and $(ANQ)$, which is perpendicular to the radical axis $AX$ of the two circles. Thus we are done.
Attachments:

12.09.2021 17:22
Claim: Consider triangle with incenter $I$ and circumcircle $\Gamma$. Take any point $D$ on $AI$ and let $(IDB)$ intersect $(ABC),AB$ at $E,F$, respectively. If $P$ is the intersection of $EF$ and the tangent from $I$ to $(IDB)$ and $S$ is the intersection of $(ABC)$ and the tangent from $B$ to $(IDB)$, prove that $A,P,S$ are collinear. Proof. [asy][asy]import olympiad; size(6cm);defaultpen(fontsize(10pt));pen org=magenta;pen med=mediummagenta;pen light=pink;pen deep=deepmagenta; pair O,A,B,C,D,E,F,I,P,Q,R,S; O=(0,0);A=dir(120);B=dir(205);C=dir(335);I=incenter(A,B,C); D=1.2I-0.2A; E=intersectionpoints(circumcircle(I,D,B),circumcircle(A,B,C))[0];F=intersectionpoints(circumcircle(I,D,B),A--B)[0]; Q=extension(E,D,I,B);P=extension(A,Q,E,F);R=extension(B,D,A,Q);S=intersectionpoints(circumcircle(A,B,C),A--10Q-9A)[1]; draw(circumcircle(A,B,C),heavyblue); draw(A--B--C--cycle,red+1);draw(circumcircle(I,D,B),heavyblue);draw(A--D,red+1);draw(P--S,heavygreen);draw(P--E,heavygreen);draw(P--I,heavygreen);draw(D--E,red);draw(B--I,red);draw(B--D,red);draw(E--I,red);draw(P--A,heavygreen); dot("$A$",A,dir(A)); dot("$B$",B,dir(B)); dot("$C$",C,dir(C)); dot("$D$",D,dir(D)); dot("$I$",I,dir(I)); dot("$E$",E,dir(E)); dot("$F$",F,dir(F)); dot("$P$",P,dir(P)); dot("$Q$",Q,dir(Q)); dot("$R$",R,dir(R)); dot("$S$",S,dir(S)); [/asy][/asy] Let $Q=IB \cap DE$ and let $R=BD\cap AP$. Let $S'$ be the intersection of $AP$ and the tangent from $B$ to $(IDB)$, we claim that $S\equiv S'$, i.e. $S'$ lies on $(ABC)$. Firstly, by Pascal's theorem on $IIBFED$, we obtain that $A,P,Q$ are collinear. Note that $AIQE$ is cyclic, because $$\measuredangle QEA=\measuredangle BEA-\measuredangle BED=\measuredangle BCA-\measuredangle BID=\measuredangle QIA.$$Furthermore, we get that $AERD$ is cyclic, because $\measuredangle EDR=\measuredangle EIQ=\measuredangle EAR$. Hence, $$\measuredangle BRS'=\measuredangle DRA=\measuredangle QEA=\measuredangle BID=\measuredangle S'BR\implies \measuredangle AS'B=2\measuredangle DIB=\measuredangle ACB.$$The claim follows. $\square$ By the claim, all we need is to show that $Y$ lies on $(ABC)$. Indeed as $(BIP)$ and $(CIQ)$ are tangent to each other, \begin{align*} \measuredangle BYC=\measuredangle BO_1I+\measuredangle IO_2C=2\measuredangle BPI+2\measuredangle IQC=2\measuredangle BIC=\measuredangle BAC, \end{align*}we are done. [asy][asy]import geometry; size(7cm);defaultpen(fontsize(10pt));pen org=magenta;pen med=mediummagenta;pen light=pink;pen deep=deepmagenta; pair O,A,B,C,K,P,M,I,O1,O2,Q,N,X,Y; O=(0,0);A=dir(120);B=dir(205);C=dir(335);I=incenter(A,B,C); K=1.1I-0.1A; P=intersectionpoints(circumcircle(I,K,B),circumcircle(A,B,C))[0];M=intersectionpoints(circumcircle(I,K,B),A--B)[0]; O1=circumcenter(I,K,B);O2=intersectionpoint(line(O1,I),perpendicular(midpoint(I--C),line(I,C))); Q=intersectionpoints(circumcircle(A,B,C),circle(O2,abs(O2-I)))[0];N=intersectionpoints(A--C,circle(O2,abs(O2-I)))[0];X=extension(P,M,Q,N);Y=intersectionpoints(circumcircle(A,B,C),A--100X-99A)[1]; draw(circumcircle(A,B,C),heavyblue); draw(A--B--C--cycle,red+1);draw(circumcircle(I,P,B),heavyblue);draw(A--I,red+1); draw(circumcircle(I,Q,C),heavyblue);draw(B--Y--C,heavygreen+1);draw(A--Y,dashed+orange+0.5); draw(P--X--Q,red+0.5);draw(I--X,red+0.5);draw(B--O1--O2--C,lightblue+0.5); dot("$A$",A,dir(A)); dot("$B$",B,dir(B)); dot("$C$",C,dir(C)); dot("$I$",I,dir(I)); dot("$P$",P,dir(P)); dot("$M$",M,dir(M)); dot("$O_1$",O1,dir(O1)); dot("$O_2$",O2,dir(O2)); dot("$Q$",Q,dir(Q)); dot("$N$",N,dir(N)); dot("$X$",X,dir(X)); dot("$Y$",Y,dir(Y)); [/asy][/asy]
18.10.2023 06:55
Let $T_1=\omega_B\cap BC$ and $T_2=\omega_C\cap BC$. We make the following claims: Claim: $Y$ lies on $\Gamma$ Proof: We angle-chase this, getting: \begin{align*} \angle T_1IT_2&=\angle T_1MI+\angle T_2NI\\ &=\angle T_2CI+\angle T_1BI\\ &=90^{\circ}-\frac{\angle A}{2}.\\ \end{align*} Therefore: \begin{align*} \angle A&=\angle BIT_1+\angle CIT_2\\ &=\angle BMT_1+\angle CNT_2\\ &=\angle YBC+\angle YCB\\ &=180^{\circ}-\angle BCY.\\ \end{align*} As a result $ABYC$ is cyclic $\blacksquare$ Claim: $T_1T_2NM$ is an isosceles trapezoid Proof: We can angle chase to see: \[\angle IT_1M=\angle IBM=\angle IBT_1=\angle IMT_1,\]so $\triangle{IT_1M}$ is isosceles. Similarly, $\triangle{IT_2M}$ is isosceles. As the centers of the two circles are collinear with $I$, we have our desired result $\blacksquare$ Claim: $PMNQ$ is cyclic Proof: We angle chase: \begin{align*} \angle MNQ&=\angle MNT_2+T_2NQ\\ &=MNT_2+180^{\circ}-\angle BCQ\\ &=180^{\circ}-\angle MT_1T_2+\angle BPQ\\ &=\angle BT_1M+\angle BPQ\\ &=180^{\circ}-\angle BPM+\angle BPQ\\ &=180^{\circ}-\angle MPQ,\\ \end{align*}as desired $\blacksquare$ Claim: $AY$ is tangent to both $(AMP)$ and $(ANQ)$ Proof: For $(AMP)$, we angle chase, getting: \begin{align*} \angle BAY&=\angle BCY\\ &=\angle NCB+\angle BCY-\angle BCA\\ &=\angle NQT_2+\angle T_2QC-\angle BCA\\ &=\angle NQC-\angle BCA\\ &=180^{\circ}-\angle NT_2C-\angle BCA\\ &=\angle BT_1M-\angle BCA\\ &=180^{\circ}-\angle BCA-\angle BPM\\ &=\angle BPA-\angle BPM\\ &=MPA,\\ \end{align*}as desired. We use the exact same approach for $(ANQ)$ and we get the same result $\blacksquare$ Using the radical center on $(PMNQ),(AMP),(ANQ)$, it is $X$. This means that $XA$ is tangent to $(ANQ)$ and $(AMP)$, so $A,X,Y$ are collinear $\square$
10.11.2023 05:36
okay this is doable with nothing but radical axis and angle chase Angle chase: Y lies on ABC. Draw PM cap ABC and QN cap ABC = U, V. Angle chase. UAV = BYC. Find angles UV, BC and MN, BC. UV and MN are parallel (this angle chase takes a while, but it does work). Now PMQN is cyclic by Reim's. Angle chase: APM and AQN are tangent. So MAX = APM, but angle chasing gives APM = BAY. Ta-da!!!!
31.12.2023 22:34
First denote $S = \omega_B \cap BC$ and $T = \omega_C \cap BC$. We claim $Y$ lies on $\Gamma$, as \[90 + \frac{\angle A}{2} = \angle BIC = \angle IBY + \angle ICY \implies \angle BYC = 180 - \angle A.\] We also note $MNTS$ is an isosceles trapezoid by symmetry about $O_BO_C$. Thus $\triangle XMN \sim \triangle XQP$ since \[\measuredangle NMX = \measuredangle NMS + \measuredangle SMP = \measuredangle NQC + \measuredangle CQP = \measuredangle XQP.\] As a result, $AX$ forms the radical axis of $(APM)$ and $(AQN)$, so we must show $Y$ lies on this radical axis. In fact, we claim $AY$ is tangent to $(APM)$, as \[\measuredangle APM = \measuredangle APB + \measuredangle BPM = \measuredangle ACB + \measuredangle YCN = \measuredangle YCB = \measuredangle YAM,\] and analogously $AY$ is also tangent to $(AQN)$. $\blacksquare$
13.01.2024 22:38
WHAT A PROBLEM. My first recent-year G8 solve!!! Solved with a few hints (specifically to arrive at Claim 2 and construct $(AMP)$ and $(ANQ)$), but still very awesome. Claim: $Y$ lies on $\Gamma.$ Proof: Let $Z$ be an arbitrary point on the tangent at $I$ to both circles closer to $Y$ than $A.$ Then we have $\measuredangle ZIB = \measuredangle IMB = \measuredangle IBY$ and $\measuredangle CIZ = \measuredangle CNI = \measuredangle YCI.$ Therefore, adding these two gives $$\measuredangle CIB = \measuredangle IBY + \measuredangle YCI.$$Consequently, because the sum of the angles in a quadrilateral is $360$ degrees, we get $$\measuredangle BYC = 2 \measuredangle BIC = \measuredangle BAC,$$as claimed. Claim 2: Let $\omega_B$ and $\omega_C$ intersect side $BC$ again at $F \ne B$ and $G \ne C,$ respectively. Then $MFGN$ is an isosceles trapezoid. Proof: Note that $I$ is the arc midpoint of minor arc $MF$ of $\omega_B$ since $\measuredangle MBI = \measuredangle IBF.$ Thus $M$ and $F$ are symmetric about the line connecting the centers of $\omega_B$ and $\omega_C,$ as are $N$ and $G.$ Furthermore, we also have that $MF$ is parallel to the tangent at $I$ on both circles, as is $NG,$ so $MF \parallel NG.$ This is enough to imply that $MFGN$ is an isosceles trapezoid, as claimed. Claim 3: $P,M,N,Q$ are concyclic. Proof: First, we can write $$\measuredangle PMN = \measuredangle PMB + \measuredangle BMF + \measuredangle FMN.$$We now note that $\measuredangle PMB = \measuredangle PBY$ since $BY$ is tangent to $\omega_B.$ We similarly note that $\measuredangle BMF = \measuredangle YBF = \measuredangle YBC.$ Finally, by Claim 2, we can write $\measuredangle FMN = \measuredangle GFM = \measuredangle BFM = \measuredangle YBM = \measuredangle YBA$. Therefore, $$\measuredangle PMN = \measuredangle PBY + \measuredangle YBC + \measuredangle YBA = \measuredangle PBC + \measuredangle YBA.$$Now, $$\measuredangle PQN = \measuredangle PQC + \measuredangle CQN = \measuredangle PBC + \measuredangle YCN = \measuredangle PBC + \measuredangle YCA.$$However, by Claim 1, $Y$ lies on $\Gamma,$ so $\measuredangle YBA = \measuredangle YCA.$ This readily implies $\measuredangle PMN = \measuredangle PQN,$ as claimed. Let $\omega$ be the common circle of $P,M,N,Q$ as proven to exist by Claim 3. Now we will finish the problem. First, since $Y$ lies on $\Gamma$ and $YB$ is tangent to $\omega_B,$ we note that $$\measuredangle YAP = \measuredangle YBP = \measuredangle BMP = \measuredangle AMP,$$so $YA$ is tangent to $(AMP).$ Similarly, $YA$ is tangent to $(ANQ).$ This means that $(PMA)$ and $(QNA)$ are tangent, with $Y$ lying on the common external tangent. On the other hand, we can apply the radical center theorem on $(PMA), (QNA),$ and $\omega$ to get that $PM \cap QN = X$ lies on this external tangent. Therefore, $A,X,Y$ are all collinear on the common external tangent of $(PMA)$ and $(QNA),$ as desired.
06.02.2024 22:04
Let $K$ be the intersection point of the tangent at $I$ and $\Gamma$. Then $\angle BIC = \angle BIK + \angle CIK = \angle IMB + \angle CNI = \angle YCI + \angle YBI$. Note that $\angle BIC = 90^{\circ} + \frac{\angle A}{2}$ so $\angle BYC = 360^{\circ} - (180^{\circ} - \angle A) = 180^{\circ} - \angle A \implies ABYC$ is cyclic. Notice that $MNC_1B_1$ is an isosceles trapezoid due to the fact that the perpendicular bisectors of $MB_1$ and $NC_1$ coincide. Note that $PBCQ$ is cyclic. We can then prove that $\triangle XMN \sim \triangle XQP$. $\angle XNM = 180^{\circ} - (\angle MNC_1 + \angle C_1NQ) = 180^{\circ} - (\angle MBB_1 + 180^{\circ} - \angle C_1CQ)$ $= \angle BPM + \angle C_1CQ - 180^{\circ} = \angle BPM - \angle BPQ = \angle XPQ$. By using similar triangle ratios, we find that $XM \cdot XP = XN \cdot XQ$, so $AX$ is the radical axis of $\omega_B$ and $\omega_C$. Similarly, we have $XN \cdot XQ = XM \cdot XP \implies AX$ is the radical axis of $(AMP)$ and $(ANP)$. All that is left is to show that $AY$ is the radical axis of $(AMP)$ and $(ANP)$ which is equivalent to $AY$ tangent to both circle. Notice that $PAYB$ is cyclic. Then $\angle PMB = 180^{\circ} - PBY = \angle PAY$, which proves the tangency, so we are done.
18.02.2024 10:27
Angle chase We use directed angles mod $180^\circ$ throughout this solution.
mfw g8 is just a massive chase
21.05.2024 01:02
Take an inversion about I. In the inverted image, we have $I$ is the orthocenter of $\triangle ABC$, $B,P,M$ collinear and $C,Q,N$ collinear. Note that \[\angle APM=\angle ACB=180^\circ-\angle AIB=\angle AMP\]so $AP=AM$ and similarly $AQ=AN$. This implies that $PMNQ$ is an isosceles trapezoid and $(MAP)$ is tangent to $(NAQ)$. In our uninverted image, note that since $PMNQ$ is cyclic, $X$ is on the radical axis of $\omega_B$ and $\omega_C$. Furthermore, it is on the radical axis of $(MAP)$ and $(NAQ)$. It is also on the common tangents from $I$ to $\omega_B$ and $\omega_C$ and from $A$ to $(MAP)$ and $(NAQ)$. Note that \[\angle BIC+\angle MIN=180^\circ-\angle IBC-\angle ICB+\angle MIX+\angle XIN=180^\circ-\angle IBC-\angle ICB+\angle IBM+\angle ICN=180^\circ\]so $\angle BIM+\angle CIN = 180^\circ\implies \angle BPM+\angle CQN=180^\circ\implies \angle ABY+\angle ACY=180^\circ$, so $Y$ is on $(ABC)$. Furthermore, \[\angle AQN=\angle AQC-\angle NQC=180^\circ-\angle AYC-\angle ACY=\angle YAC\]so $YA$ is tangent to $(NAQ)$. Similarly, it is tangent to $(MAP)$ and so $Y$ is also on the common tangent from $A$ to $(MAP)$ and $(NAQ)$. We are done.
06.08.2024 04:54
Invert about $I$ and rename $I$ to $H$ to get the following equivalent statement: Let $\triangle ABC$ have orthocenter $H$. Let $\ell_B$, $\ell_C$ be paralell lines through $B$ and $C$, respectively. Let $P$ and $Q$ be the intersections of $\ell_B$ and $\ell_C$ with $(ABC)$, respectively. Let $M$ and $N$ be the intersections of $\ell_B$ and $\ell_C$ with $(AHB)$ and $(AHC)$, respectively. Let $X$ be the intersection of $(HPM)$ and $(HQN)$. Let $Y$ be the second intersection of the circle through $H$ tangent to $\ell_B$ at $B$ and the circle through $H$ tangent to $\ell_C$ at $C$. Then, $AHYX$ is cyclic. We will prove this statement instead of the original. Let $H'$ be the reflection of $H$ over the angle bisector of $BC$ and $PQ$. Then, $\triangle BHC \cong PH'Q$. Claim: $X$ is the intersection of $HH'$ with $(H'PQ)$. Proof: We have \[\angle PXQ = \angle PXH + \angle HXQ = \angle BMH + \angle HNC\]\[= \angle BAH + \angle HAC = \angle BAC = \angle CHB = \angle PH'Q.\]Thus, $X$ lies on $(PH'Q)$. We have \[\angle PXH = \angle PMH = \angle BMH = \angle BAH = \angle HCB = \angle PQH' = \angle PXH'.\]Thus, $X$ is on $HH'$. Claim: $A$ is the orthocenter of $\triangle PXQ$. Proof: Let $D$ be the intersection of $PQ$ and $HH'$. We have \[\angle APQ = \angle ACQ = \angle BCQ + \angle ACB\]\[\angle PQX = \angle DXQ + \angle QDX = \angle HCB + \angle QCB.\]Thus, \[\angle APQ + \angle PQX = \angle BCQ + \angle QCB + \angle ACB + \angle HCB = 90.\]Repeating the same logic on the other side, $A$ is the orthocenter of $\triangle PXQ$. Claim: $AHYX$ is cyclic Proof: We have \[\angle AXH = \angle AXP + \angle PXH' = \angle PQA + \angle PQH'\]\[= \angle PBA + \angle HCB = \angle PBC + \angle CBA + \angle HCB = \angle PBC + 90\]\[= \angle PBC + \angle CBH + \angle ACB = \angle PBH + \angle ACB = \angle BYH + \angle AYB\]\[= \angle AYH.\]
27.08.2024 13:06
Eyed wrote: Let $ABC$ be a triangle with incenter $I$ and circumcircle $\Gamma$. Circles $\omega_{B}$ passing through $B$ and $\omega_{C}$ passing through $C$ are tangent at $I$. Let $\omega_{B}$ meet minor arc $AB$ of $\Gamma$ at $P$ and $AB$ at $M\neq B$, and let $\omega_{C}$ meet minor arc $AC$ of $\Gamma$ at $Q$ and $AC$ at $N\neq C$. Rays $PM$ and $QN$ meet at $X$. Let $Y$ be a point such that $YB$ is tangent to $\omega_{B}$ and $YC$ is tangent to $\omega_{C}$. Show that $A,X,Y$ are collinear. We'll prove this problem follows these claims : $\textbf{1/}$ Let $B'$ be the intersection of line $BI$ with $\omega_c$ ($B' \neq I$); $C'$ be the intersection of line $CI$ with $\omega_b$ ($C' \neq I$) $\hspace{1cm}$$M'$ be the intersection of lines $C'M$ and $AC$; $N'$ be the intersection of lines $B'N$ and $AB$ Then $N'MM'N$ is a isosceles trapezoid and $I$ lie on circumcircle of $N'MM'N$ $\textit{Prove :}$ $\, \,$Cuz $M$, $I$, $B$, $C$ are concyclic and $I$ is incenter of $\triangle ABC$, we see that : $$ \angle AM'M = \angle MC'C + \angle ACC' = \angle IBC + \angle ICB = \angle C'IB = \angle C'MB $$$\, \,$So $\triangle AMM'$ is a isosceles triangle at $A$. Similar, $\triangle ANN'$ is a isosceles triangle at $A$ $\, \,$Therefore, $N'MM'N$ is a isosceles trapezoid $\, \,$Next, let $\ell$ be the line which tangent both $\omega_b$, $\omega_c$ at $I$, we see that : $$ \angle MIN = \angle MI\ell + \angle NI\ell = \angle MBI + \angle NCI = \angle IBC + \angle ICB = \angle AM'M $$$\, \,$Therefore, $M'$ lies on circle $(IMN)$. Similar, $N'$ lies on circle $(IMN)$. $\textbf{2/}$ $M$, $N$, $P$, $Q$ are concyclic $\textit{Prove :}$ $\, \,$Let $\omega_b$, $\omega_c$ meet line $BC$ at $T$, $L$ respectively ($T \neq B$, $L \neq C$) $\hspace{1cm}$$O_b$, $O_c$ be center of $\omega_b$, $\omega_c$ respectively $\, \,$Cuz $CI$ is the internal bisector of $\angle NCL$ so $O_cI \perp NL$. Similar, $O_bI \perp MT$ $\, \,$But $O_b$, $I$, $O_c$ are colibnear, so $NL$ $\|$ $MT$ $\, \,$Combine with claim $\textbf{1/}$, we see that : $$\angle N'IN = 2\angle NIA = 2(\angle AIC - \angle NIC) = 180^\circ + \angle BAC - 2\angle NIC = \angle BAC + \angle NQC - \angle NLC = \angle NQC - \angle BMT$$$\, \,$So $$ \angle PMN = \angle PMA + \angle AMN = \angle PC'B + \angle N'IN = \angle PBT + \angle NQC = 180^\circ - (\angle PQC - \angle NQC) = 180^\circ - \angle PQN$$$\, \,$Therefore, $M$, $N$, $P$, $Q$ are concyclic $\textbf{3/}$ Let $V$ be the midpoint of arc $BAC$ of $\Gamma$ Then $V$ is the intersection of lines $B'Q$, $C'P$ $\textit{Prove :}$ $\, \,$Cuz $Q$ is the second intersection of $\omega_c$ and $\Gamma$ so $\angle B'QC = \angle BIC = 90^\circ + \frac12 \angle BAC = \angle VQC$ $\, \,$Lead to $V$, $B'$, $Q$ are colinear. Similar, $P$, $C'$, $P$ are colinear $\textbf{4/}$$\triangle AXI$ is a isosceles triangle at $X$ $\textit{Prove :}$ $\, \,$Let $V_b$, $V_c$ be the intersections of lines $BI$, $CI$ with $\Gamma$ respectively ($V_b \neq B$, $V_c \neq C$) $\hspace{0.7cm}$$U_b$ be the intersection of lines $V_bQ$ and $B'N$ $\, \,$We ez to see that $V_bV_c$ is the perpendicular bisector of $AI$, so we just need to prove that $V_b$, $X$, $V_c$ are colinear $\, \,$Cuz $M$, $N$, $P$, $Q$ are concyclic from $\textbf{2/}$, so $X$ lie on radical axis of $\omega_b$ and $\omega_c$ $\, \,$Which means $XI$ tangent with both $\omega_b$, $\omega_c$ at $I$ $\, \,$Lead to $\triangle XNI \sim \triangle XIQ$. As a result : $\dfrac{XN}{XQ} = \dfrac{XN}{XI} .\dfrac{XI}{XQ} = \left( \dfrac{IN}{IQ} \right)^2 $ $\, \,$In the order hand : $\hspace{0.5cm}$From claim $\textbf{3/}$ and $NB' \perp AI$ at claim $\textbf{1/}$, we see that : $$\angle V_bQB' = \angle V_bQV = 90^\circ - \frac12(\angle BAC + \angle ABC) = 90^\circ - \angle AIB' = \angle NB'I = \angle V_bB'U_b$$$\hspace{0.5cm}$Lead to $\triangle V_bU_bB' \sim \triangle V_bB'Q$. As a result : $\dfrac{V_bU_b}{V_bQ} = \dfrac{V_bU_b}{V_bB'} .\dfrac{V_bB'}{V_bQ} = \left( \dfrac{B'U_b}{B'Q} \right)^2 $ $\, \,$Therefore $$ \dfrac{XN}{XQ} = \left( \dfrac{IN}{IQ} \right)^2 = \left( \dfrac{\sin \angle NQI}{\sin \angle INQ} \right)^2 = \left( \dfrac{\sin \angle V_bB'U_b}{\sin \angle V_bB'Q} \right)^2 = \left( \dfrac{V_bU_b}{B'V_b} .\dfrac{V_bB'}{V_bQ} \right)^2 = \dfrac{V_bU_b}{V_bQ} $$$\, \,$Hence, follow Thales theorem, we see that $XV_b$ $\|$ $NB'$. Similar, $XV_c$ $\|$ $MC'$ $\, \,$But from claim $\textbf{1/}$, we already have that $B'N$ $\|$ $C'M$, so $X$, $V_b$, $V_c$ are colinear $\textbf{Finally, from these claims, we have : }$ $\, \,$ Cuz $\triangle XMI \sim \triangle XIP$ and $AX = XI$, so $\triangle XMA \sim \triangle XAP$ $\, \,$Lead to $$\angle MAX = \angle APM = \angle APB - \angle MPB = 180^\circ - \angle ACB - \angle MTC = \angle LNC = \angle BCY = \angle BAY $$$\, \,$Similar, $\angle XAN = \angle CAY$. Therefore $A$, $X$, $Y$ are colinear, done
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imosl g8 2020.pdf (74kb)
29.08.2024 18:54
Claim: $Y$ lies on $(ABC)$. Proof: \[\measuredangle BYC= 180-\measuredangle CBY-\measuredangle YCB=180-(\measuredangle IMB-\frac{\measuredangle B}{2})-(\measuredangle CNI-\frac{\measuredangle C}{2})=270-\frac{\measuredangle A}{2}-\measuredangle CIB=180-\measuredangle A\]Which gives that $Y\in (ABC)$.$\square$ Claim: $I$ is the $A-$excenter on $\triangle AMN$. Proof: Let $O$ be the circumcenter of $(IMN)$. Since $\measuredangle MIN=\measuredangle MBI+\measuredangle ICN=90-\frac{\measuredangle A}{2},$ we have $\measuredangle MON=180-\measuredangle MAN$ hence $O$ lies on $(AMN)$. If $AI$ intersects $(IMN)$ at $J$ for second time, then \[\measuredangle IMB=\measuredangle MIJ+\frac{\measuredangle A}{2}=\measuredangle MNJ+\measuredangle ONM=\measuredangle ONJ=\measuredangle NJO=\measuredangle NMI\]Similarily we get $\measuredangle CNI=\measuredangle INM$ so $I$ is the $A-$excenter on $\triangle AMN$.$\square$ Claim: $M,N,P,Q$ are concyclic. Proof: Let $MN$ and incircle of $ABC$ be tangent to each other at $S$. Take the inversion centered at $I$ with radius $IS$. Let $D,E,F$ be the tangency points of the incircle with $BC,CA,AB$ respectively. $A^*,B^*,C^*,M^*,N^*$ are the midpoints of $EF,FD,DE,SF,SE$. $P^*,Q^*$ are the intersections of $B^*M^*,C^*N^*$ with $(A^*B^*C^*)$. Note that $M^*B^*\parallel SD\parallel N^*C^*$ and $M^*N^*\parallel EF\parallel B^*C ^*$. \[\measuredangle M^*P^*Q^*=180-\measuredangle Q^*P^*B^*=\measuredangle B^*C^*N^*=\measuredangle N^*M^*P^*\]Thus, $N^*M^*P^*Q^*$ is an isosceles trapezoid which yields $M,N,P,Q$ lie on a circle.$\square$ Claim: $A,X,Y$ are collinear. Proof: Let $PM$ and $QN$ intersect $(ABC)$ at $K,L$ for second time. \[\measuredangle YCA=\measuredangle YCN=\measuredangle CQN=\measuredangle CQL \]\[\measuredangle ABY=\measuredangle MBY=\measuredangle MPB=\measuredangle KPB\]These give that $AY=CL$ and $AY=BK$. Hence $YK\parallel AB$ and $YL\parallel AC$. Also \[\measuredangle KLN=\measuredangle KLQ=\measuredangle KPQ=\measuredangle MPQ=\measuredangle MNL\]Thus, $MN\parallel KL$. By Desargues on $\triangle AMN$ and $\triangle YKL$, since $AM\cap YK=AB_{\infty},MN\cap KL=MN_{\infty},NA\cap LY=AC_{\infty}$ are collinear, they are perspective. $AY,MK,NL$ are concurrent. $MK$ and $NL$ intersect at $X$ so $A,X,Y$ are collinear.$\blacksquare$
12.10.2024 01:23
Why did I solve a G8 synthetically??? Let's denote with $t$ the common tangent of $w_b$ and $w_c$. Also let $U,V$ be the intersections of $BC$ with $w_b$ and $w_c$ Claim 1: $Y$ lies on $(ABC)$ Proof. $\angle BIU=\angle BCY=\phi$ and $\angle CIV=\angle CBY=\psi, \angle UIt=\angle IBC=\frac{\beta}{2}, \angle VIt=\angle ICB=\frac{\gamma}{2}$ Now: $$90+\frac{\alpha}{2}=\angle BIC=\angle BIU+\angle UIV+\angle CIV= \phi+\psi+90-\frac{\alpha}{2}$$So $\angle BYC=180-\phi-\psi=180- \alpha=\angle BAC \text{. }\square$ Claim 2: $MU$ || $NV$ || $t$ Proof. $\angle MUI=\angle MBI= \angle UBI = \angle UIt \Rightarrow $ $MU$ || $t$ and similarly $NV$ || $t$. $\square$ Claim 3: $MUVN$ is an isosceles trapezoid Proof. As $I$ is the midpoint of arcs $MU$ and $NV$ in $w_b,w_c$, this means that $MI=UI, IN=IV \Rightarrow \triangle MIN$ and $\triangle UIV $ are congruent $\Rightarrow MN=UV$. $\square$ Let $Q'$ and $P'$ be the intersections of $QX$ and $PX$ with $(ABC)$. Claim 4: $BQ'$ || $QC$ || $t$ Proof. Reims theorem on $(ABC), (NQCV)$ and lines $QX,CV$ gives that $BQ'$ || $NV$ and similarly $CP'$ || $UM$. $\square$ Let $K;G$ be the intersections of $AB;BC$ and $NV;MU$ Claim 5: $PMAG$ and $QNAK$ are cyclic Proof. $\angle KNQ=\angle VCQ=180-\angle BQ'Q=180-\angle BAQ=\angle KAQ$ and similarly $\angle PMG=\angle PAG$. $\square$ Claim 6: $(PMAG)$ and $(NAKQ)$ are tangent at $A$ Proof. As $GMNK$ is a trapezoid we have that the circumcenters of $GMA$ and $KAN$ lie on a line with $A$. $\square$ Claim 7: $PMNQ$ is cyclic Proof. $\angle MPQ= \angle BPQ-\angle BPM=(180-\angle BCQ)-\angle MUV=$ $(180-\angle KNQ)-\angle UMN=\angle XNK-\angle MNK=\angle MNX$. $\square$ Claim 8: $A,X,Y$ are collinear Proof. By radical axises on $(PMNQ), (PMAG), (NAKQ)$ we get that $AX$ is the tangent of $(PMAG)$ and $(NAKQ)$. $\angle YAC=\angle YBC= \angle BMU=\angle Q'BM=\angle Q'QA=\angle XAC$. $\square$
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12.11.2024 15:20
Amazing problem! also my first G8 We show $AY$ is tangent to $\odot(AMP), \odot(ANQ)$ $$\angle YAM+\angle MAP = 180^{\circ} - \angle YBP=180^{\circ}-\angle PBI-\angle BPI = \angle BIP = 180^{\circ} - \angle PMA$$Note that $Y\in{\odot(ABC)}$as \begin{align*} & \angle BIC=\angle IMB+\angle INC=\angle IBY+\angle ICY=90^{\circ}+\frac{\angle A}{2}\\ & \angle BYC=360-\angle YBI-\angle BIC-\angle ICY=360^{\circ}-2\left(90^{\circ}+\frac{\angle A}{2}\right)=180^{\circ}-\angle A \end{align*}By another angle chase we show $\odot(I)$ is the incircle of $MNCB$. $$\angle MIN = \angle MBI + \angle NCI = \angle IBC + \angle ICB = 180^{\circ}-\angle BIC=90-\frac{\angle A}{2}$$If we show $\odot(MPNQ)$ we get $X$ as the radical center of $(MNQP)$, $(AMP)$, and $(ANQ)$, which will clearly lie on radax of $(AMP)$, and $(ANQ)$ which is clearly $\overline{AY}$ implying $\overline{A-X-Y}$ is collinear. So finally we go on to another angle chase to prove $\odot(MPNQ)$ which took me a lotta time to figure out \begin{align*} \angle PQN=\angle CQN-\angle CQP&= 180^{\circ}-\angle CIN-\angle CQP\\&=180^{\circ}-(180^{\circ}-\angle INC-\angle ICN)-\angle CQP\\&=180^{\circ}-(180^{\circ}-\angle ICY-\angle ICN)-\angle CQP\\&=\angle ICY+\angle ICN-\angle CQP\\&=\angle ACY-\angle CQP\\&=(180^{\circ}-\angle ABY)-(180^{\circ}-\angle CBP)\\&=\angle CBP-\angle ABY\\&=\angle CBA+\angle ABP-\angle CBA-\angle YBC\\&=\angle ABP-\angle YBC\\ \angle PMN=\angle PMB+\angle BMN&=\angle PIB+2\angle BMI\\&=180^{\circ}-(\angle IPB+\angle PBI)+2\angle BMI\\&=180^{\circ}-(\angle IBY+\angle PBI)+2\angle BMI\\&=180^{\circ}-\angle PBY+2\angle BMI\\&=180^{\circ}-\angle PBY+2\angle CBY+\angle CBA\\&=180^{\circ}-\angle PBA+\angle CBY \end{align*}and we are finally done!
12.11.2024 18:20
Here's my solution (angle-chase+invert+radical-axis+trig) : Claim-1: $Y \in (ABC)$ Notice that: $$\angle ABY+\angle ACY = 360^\circ - (\angle MIB + \angle NIC) = \angle MIC + \angle BIC = \angle MIX+\angle XIN + 90^\circ + \angle A/2$$$$ = \angle MBI + \angle NCI + 90^ + \angle A/2 = 180^\circ.$$ Claim-2: $MNQP$ is cyclic + $(AMP), (ANQ)$ are tangent. Invert around incircle. Let $X'$ denote the inverse of point $X$ in this inversion. Notice that: -$I$ is the orthocenter of $\triangle A'B'C'$. -$M'A'B'I$ and $N'A'C'I$ are cyclic ($A, M, B$ and $A, N, C$ are collinear). -$M', P', B'$ and $N', Q', C'$ are collinear ($MPIB$ and $NIAC$ are cyclic). -$M'P'B' \parallel N'Q'C'$. -$A',P',B',C',Q'$ lie on same circle. We will prove $M'N'Q'P'$ is cyclic. It suffice to show $M'N'=P'Q'$. Note that: $\angle M'IB'+\angle N'IC' = 360^\circ - (\angle B'M'I + \angle C'N'I) - (\angle M'B'I + \angle N'C'I) = 180^\circ.$ Now, we have: $$M'B' = \frac{B'I \sin \angle M'IB'}{\sin \angle M'B'I}$$Note that: $$\frac{B'I}{\sin \angle M'B'I} = 2R = \frac{C'I}{\sin \angle N'C'I}.$$Thus, we have: $\frac{B'I \sin \angle M'IB'}{\sin \angle M'B'I} = \frac{C'I \sin \angle N'IC'}{\sin \angle N'C'I} = N'C'$ which implies $M'B'C'N'$ is a parallelogram. Note that, from the following sub-claim, we will have: $P'Q' \parallel M'N' \parallel BC$ and $(A'M'P'), (A'N'Q')$ to be tangent at $A'$ which finishes the main-claim. Sub-Claim: $\triangle A'M'P'$ and $\triangle A'N'Q'$ are isosceles. Note that: $\angle A'M'B' = 180^\circ - \angle A'IB' = \angle A'C'B'$ and $\angle A'P'M' = \angle A'C'B'$ which shows that $A'M' = A'P'$. Similarly, $A'N'=A'P'$ and we are done. Hence, Claim-2 is true. Thus, from Claim-2, $AX$ is the radical axis of $(APM), (ANQ)$ along with $AX$ being tangent to both circles $(APM), (ANQ)$. Note that, the following claim finishes the problem: Claim-3: $AY$ is tangent to both circles $(APM), (ANQ)$. Let $R$ be a point of line $AP$ such that $A, R$ are on different sides with-respect to $P$. We will show it with an one-liner angle-chase: $$\angle BAY = 180^\circ - \angle AYB - \angle ABY = 180^\circ - \angle RPB - \angle BPM = \angle APM.$$Thus, $AY$ is tangent to $(APM)$. Similarly, $AY$ is tangent to $(ANQ)$ and we are done.
24.12.2024 13:50
We prove that $Y$ lies on $(ABC)$ through angle chasing - $\angle ABY+\angle ACY=360-\angle MIB-\angle NIC=180^\circ.$ Next, $YA$ is tangent to both $(ANQ)$ and $(AMP)$ and is thus the radical axis, as $\angle APM=\angle BPA-\angle BPM=(180-\angle BYA)-\angle YBA=\angle YAB=\angle YAM$ and the same works for $(ANQ).$ Finally, we show $X$ lies on the radical axis by showing $MNQP$ are concyclic to finish. Let $\omega_b$ and $\omega_c$ intersect $BC$ at $R,S$ respectively. Observe by Fact 5 $\triangle IRS \cong \triangle IMN$ so $\angle INM=\angle ISR=\angle INC.$ \begin{align*} \angle QPM &= \angle MPB-\angle QPB \\ &= \angle NIC+\angle QCB-180 \\ &= \angle NIC+\angle BCI-\angle QNI \\ &=\angle NIC+\angle NCI-\angle QNI \\ &=180-\angle INC-\angle QNI \\ &=180-\angle INM-\angle QNI \\ &=180-\angle QNM \end{align*}so we are done.
16.01.2025 00:15
Part 1 - The spiral similarity First, note that \[\angle BIC=\angle BMI+\angle CNI=\angle YBI+\angle YCI.\]Therefore, \[\angle BYC=360^{\circ}-\angle BIC-\angle BIC=180^{\circ}-\angle A.\]Therefore, $Y\in (ABC)$. Now note that \[\angle PBY=180^{\circ}-\angle PMB=\angle PMA\]\[\angle PYB=\angle PAB=\angle PAM\]so $\triangle PBY\sim\triangle PMA$. So there exists a spiral similarity centered at $P$ sending $AM$ to $YB$. Part 2 - Redefining $X$ Redefine $X$ to be the intersection of the perpendicular bisector of $AI$ with $AY$. Then $\angle IXY=2\angle IAX$, so $IX$ is parallel to the isogonal of $AY$ wrt angle $BAC$. In particular, \[\angle(BA,IX)=\angle YAC=\angle YBC=\angle(YB,CB)\Longrightarrow \angle(BI,IX)=\angle(YB,IB)\Longrightarrow \angle BIX=180^{\circ}-\angle YBI=180^{\circ}-\angle BPI.\]Therefore, $XI$ is tangent to $(BPI)$. Part 3 - The finish By our spiral similarity from earlier, \[\angle YAM=\angle YAB=\angle YPB=\angle APM.\]Therefore, $AY$ is tangent to $(APM)$. Then the powers from $X$ to $(APM)$ and $(BPM)$ are $AX^2$ and $IX^2$, respectively, which are equal, so $X$ lies on their radical axis $PM$. Similarly, $X$ lies on $QN$, as desired. $\blacksquare$