Problem

Source: ELMO Shortlist 2012, G7

Tags: geometry, circumcircle, trigonometry, geometry solved, Inversion, ELMO Shortlist, USA



Let $\triangle ABC$ be an acute triangle with circumcenter $O$ such that $AB<AC$, let $Q$ be the intersection of the external bisector of $\angle A$ with $BC$, and let $P$ be a point in the interior of $\triangle ABC$ such that $\triangle BPA$ is similar to $\triangle APC$. Show that $\angle QPA + \angle OQB = 90^{\circ}$. Alex Zhu.