MathNet 2022-part-04 — 197 problems (EN–VI) Question 84

Let $BB'$, $CC'$ be the altitudes of an acute-angled triangle $ABC$. Two circles passing through $A$ and $C'$ are tangent to $BC$ at points $P$ and $Q$. Prove that $A$, $B'$, $P$, $Q$ are concyclic.