1999 OIM Problems/Problem 5

Problem

An acute triangle $ABC$ is inscribed in a circle with center $O$.

The heights of the triangle are $AD$, $BE$ and $CF$. The line $EF$ cuts the circle at $P$ and $Q$.

a) Prove that $OA$ is perpendicular to $PQ$.

b) If $M$ is the midpoint of $BC$, prove that $AP^2 = 2. AD. OM$

~translated into English by Tomas Diaz. ~orders@tomasdiaz.com

Solution

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See also

https://www.oma.org.ar/enunciados/ibe14.htm