Given $\triangle ABC$ and points $D$ and $E$ at the line $BC$, furthermore there are points $X$ and $Y$ inside $\triangle ABC$. Let $P$ be the intersection of line $AD$ and $XE$, and $Q$ be the intersection of line $AE$ and $YD$. If there exist a circle that passes through $X, Y, D, E$, and $$\angle BXE + \angle BCA = \angle CYD + \angle CBA = 180^{\circ}$$Prove that the line $BP$, $CQ$, and the perpendicular bisector of $BC$ intersect at one point.
Problem
Source: Indonesia Regional 2023, Part II P5
Tags: geometry, Indonesia, regional olympiad, RMO
05.06.2023 15:39
you sure it's $BP, CQ, BC$? because it's impossible for them to intersect at one point
05.06.2023 16:59
i think it's perpendicular bisector of $BC$
05.06.2023 17:47
ignitedsoulv wrote: i think it's perpendicular bisector of $BC$ indeed, this is the correct statement
06.06.2023 08:27
Indonesia RMO 2023/5 wrote: Given a triangle $ABC$ and points $D$ and $E$ on $BC$. Points $X$ and $Y$ are inside $\triangle ABC$ such that \[ \angle BXE + \angle BCA = \angle CYD + \angle CBA = 180^{\circ} \]Suppose $AD$ intersects $XE$ at point $P$ and $AE$ intersects $YD$ at point $Q$. If $X,Y,D,E$ lies on a circle, prove that $BP, CQ$ and the perpendicular bisector of $BC$ concur. Literally angle chasing on steroids. Construct point $F = AD \cap (ABC), G = AE \cap (ABC), H = XE \cap AC, I = YD \cap AB$. Claim 01. $CFDY$, $CYQG$, $AXGH$ is cyclic and therefore by symmetry, $BXPF, BXEG, AYFI$ is cyclic. Proof. The first two are immediate, as $\angle QGC = \angle QGC = \angle ABC = \angle AFC = \angle DFC$ and $\angle DYC = \angle QYC = 180^{\circ} - \angle ABC$, the desired conclusion holds. For the last condition, we have $\angle BXH \equiv \angle BXE = 180^{\circ} - \angle BCA = \angle BCH$ and hence $BXCH$ is cyclic. Therefore, \[\angle XHA \equiv \angle XHC = \angle XBC \equiv \angle XBE = \angle XGE \equiv \angle XGA\]as desired. Claim 02. $XYGF$ is cyclic. Proof. To prove this, notice that \begin{align*} \angle GXY &= \angle GXH + \angle EXY \\ &= \angle GAC + \angle CDY = \angle GFC + \angle CFY = \angle GFY \end{align*} To finish the problem, we obtain \begin{align*} \angle QCB &= \angle YCD - \angle YCQ \\ &= \angle YFD - \angle YGQ \\ &= (\angle YFD - \angle YFX) - (\angle YGQ - \angle YGX) \\ &= \angle EGX - \angle XFD \\ &= \angle XBE - \angle XBP = \angle PBC \end{align*}and therefore, $BP, CQ$ and the perpendicular bisector of $BC$ concur. 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06.06.2023 09:24
Thx for the solutions and the correction