Problem

Source: 2021 Saudi Arabia Training Lists p1 https://artofproblemsolving.com/community/c2758131_2021_saudi_arabia_training_tests

Tags: geometry, tangent, excircle



Let $ABC$ be an acute, non-isosceles triangle with $AD$,$BE$, $CF$ are altitudes and $d$ is the tangent line of the circumcircle of triangle $ABC$ at $A$. The line through $H$ and parallel to $EF$ cuts $DE$, $DF$ at $Q, P$ respectively. Prove that $d$ is tangent to the ex-circle respect to vertex $D$ of triangle $DPQ$.