Problem

Source: 2017 Grand Duchy of Lithuania, Mathematical Contest p3 (Baltic Way TST)

Tags: geometry, parallel, right triangle, circumcircle



Let $ABC$ be a triangle with $\angle A = 90^o$ and let $D$ be an orthogonal projection of $A$ onto $BC$. The midpoints of $AD$ and $AC$ are called $E$ and $F$, respectively. Let $M$ be the circumcentre of $\vartriangle BEF$. Prove that $AC\parallel BM$.