Let $ABCDEF$ be a regular hexagon with sidelength s. The points $P$ and $Q$ are on the diagonals $BD$ and $DF$, respectively, such that $BP = DQ = s$. Prove that the three points $C$, $P$ and $Q$ are on a line. (Walther Janous)
Problem
Source: 2023 Austrian Mathematical Olympiad , Junior Regional Competition , Problem 2
Tags: geometry, collinear, hexagon
parmenides51
11.05.2024 21:24
Let O be the center of the regular hexagon with side a=BC.
We know that triangles OAB, OBC, OCD, ODE, OEF, OFA are equilaterals with side a=BC.
BP bisects angle <OBC= 60^o because OBCD is rhombus , so <OBD=30^o
similarly <ODQ=30^o
Draw circle with center B and radius a=BC=BO, then <OPC=(QP)/2= <BQP = 30^o
Draw circle with center D and radius a= DE= DO=DC,
then <OCQ=(OQ)/2 = <OQD= 30 ^o
so <OCQ = 30^o= <OPC which means that O, P, Q are collinear
Notation: (XY) stands for small arc XY
aidan0626
11.05.2024 22:02
[asy][asy]
pair A=(0,2sqrt(3));
pair B=(2,2sqrt(3));
pair C=(3,sqrt(3));
pair D=(2,0);
pair E=(0,0);
pair F=(-1,sqrt(3));
draw(A--B--C--D--E--F--cycle);
label("$A$",A,NW);
label("$B$",B,NE);
label("$C$",C+(0.23,0),E);
label("$D$",D,SE);
label("$E$",E,SW);
label("$F$",F,W);
draw(B--D);
draw(D--F);
pair P=(2,2sqrt(3)-2);
pair Q=(2-sqrt(3),1);
label("$P$",P,NW);
label("$Q$",Q,NE);
dot(P);
dot(Q);
draw(C--Q);
add(pathticks(D--E,2,.5,0,10));
add(pathticks(D--Q,2,.5,0,10));
add(pathticks(B--P,2,.5,0,10));
[/asy][/asy]
WLOG $s=2$, let $E=(0,0).$ We have $D=(2,0)$, $F=(-1,\sqrt3)$, $B=(2,2\sqrt3),$ and $C=(3,\sqrt3)$. Note that the slope of $DF$ is $-\frac{1}{\sqrt3}.$
$P$ is $(2,2\sqrt3-2)$ since you just go down $2$ from $B$.
$Q$ is $(2-\sqrt3,1)$ since from $D$, you go to the left $\sqrt3$ and up $1$, which is a distance of $2$ and matches the slope of $DF.$
We can now see that $C$, $P$, and $Q$ are collinear, since they all pass through the line $y=(-\sqrt3+2)x+(4\sqrt3-6).$