Skip to content / Đến nội dung
ThichHocToan.com

IMO 2024 — Complete Shortlist Question 18

IMO Shortlist · IMO 2024 — Complete Shortlist

Question 18

Let $ABC$ be a triangle with $AB \lt AC \lt BC$, incentre $I$ and incircle $\omega$. Let $X$ be the point in the interior of side $BC$ such that the line through $X$ parallel to $AC$ is tangent to $\omega$. Similarly, let $Y$ be the point in the interior of side $BC$ such that the line through $Y$ parallel to $AB$ is tangent to $\omega$. Let $AI$ intersect the circumcircle of triangle $ABC$ again at $P \neq A$. Let $K$ and $L$ be the midpoints of $AB$ and $AC$, respectively.

Prove that $ \angle KIL + \angle YPX = 180^{\circ} $.