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Difference between revisions of "2009 IMO Problems/Problem 2"
(→‎Solution)
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== Solution ==
 
== Solution ==
−
 
+
===Diagram===
 +
<asy>
 +
dot("O", (50, 43), W);
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dot("A", (40, 100), N);
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dot("B", (0, 0), S);
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dot("C", (100, 0), S);
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dot("Q", (24, 60), W);
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dot("P", (52, 80), E);
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dot("L", (62, 30), SE);
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dot("M", (38, 70), N);
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dot("K", (26, 40), S);
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draw((100, 0)--(24, 60), dotted);
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draw((0, 0)--(52, 80), dashed);
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draw((0, 0)--(100, 0)--(40, 100)--cycle);
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draw((24, 60)--(52, 80));
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draw((26, 40)--(38, 70)--(62, 30)--cycle);
 +
draw(circle((47, 48), 23));
 +
</asy>
 +
Diagram by qwertysri987
 +
----------------------------
 
By parallel lines and the tangency condition, <cmath>\angle APM\cong \angle LMP \cong \angle LKM.</cmath> Similarly, <cmath>\angle AQP\cong \angle KLM,</cmath> so AA similarity implies <cmath>\triangle APQ\sim \triangle MKL.</cmath> Let <math>\omega</math> denote the circumcircle of <math>\triangle ABC,</math> and <math>R</math> its circumradius. As both <math>P</math> and <math>Q</math> are inside <math>\omega,</math>  
 
By parallel lines and the tangency condition, <cmath>\angle APM\cong \angle LMP \cong \angle LKM.</cmath> Similarly, <cmath>\angle AQP\cong \angle KLM,</cmath> so AA similarity implies <cmath>\triangle APQ\sim \triangle MKL.</cmath> Let <math>\omega</math> denote the circumcircle of <math>\triangle ABC,</math> and <math>R</math> its circumradius. As both <math>P</math> and <math>Q</math> are inside <math>\omega,</math>  
  

Revision as of 23:18, 21 July 2020

Problem

Let $ABC$ be a triangle with circumcentre $O$. The points $P$ and $Q$ are interior points of the sides $CA$ and $AB$ respectively. Let $K,L$ and $M$ be the midpoints of the segments $BP,CQ$ and $PQ$, respectively, and let $\Gamma$ be the circle passing through $K,L$ and $M$. Suppose that the line $PQ$ is tangent to the circle $\Gamma$. Prove that $OP=OQ$.

Author: Sergei Berlov, Russia

Solution

Diagram

[asy] dot("O", (50, 43), W); dot("A", (40, 100), N); dot("B", (0, 0), S); dot("C", (100, 0), S); dot("Q", (24, 60), W); dot("P", (52, 80), E); dot("L", (62, 30), SE); dot("M", (38, 70), N); dot("K", (26, 40), S); draw((100, 0)--(24, 60), dotted); draw((0, 0)--(52, 80), dashed); draw((0, 0)--(100, 0)--(40, 100)--cycle); draw((24, 60)--(52, 80)); draw((26, 40)--(38, 70)--(62, 30)--cycle); draw(circle((47, 48), 23)); [/asy] Diagram by qwertysri987

By parallel lines and the tangency condition, \[\angle APM\cong \angle LMP \cong \angle LKM.\] Similarly, \[\angle AQP\cong \angle KLM,\] so AA similarity implies \[\triangle APQ\sim \triangle MKL.\] Let $\omega$ denote the circumcircle of $\triangle ABC,$ and $R$ its circumradius. As both $P$ and $Q$ are inside $\omega,$

\begin{align*} R^2-QO^2&=\text{Pow}_{\omega}(Q)\\ &=QB\cdot AQ \\ &=2AQ\cdot MK\\ &=2AP\cdot ML\\ &=AP\cdot PC\\ &=\text{Pow}_{\omega}(P)\\ &=R^2-PO^2. \end{align*} It follows that $OP=OQ.$ $\blacksquare$

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