Question 11
Let $ABC$ be a triangle inscribed in circle $(O)$, with orthocenter $H$. Let $d$ be an arbitrary line which passes through $H$ and intersects $(O)$ at two points $P$ and $Q$. Draw diameter $AA'$ of circle $(O)$. $A'P$, $A'Q$ meet $BC$ at $K$, $L$, respectively. Prove that $O$, $K$, $L$, $A'$ are concyclic.