Problem

Source: 8th European Mathematical Cup Senior Category Problem 03.

Tags: geometry, concurrency, incenter, circumcircle, mixtilinear incircle



In an acute triangle $ABC$ with $|AB| \not= |AC|$, let $I$ be the incenter and $O$ the circumcenter. The incircle is tangent to $\overline{BC}, \overline{CA}$ and $\overline{AB}$ in $D,E$ and $F$ respectively. Prove that if the line parallel to $EF$ passing through $I$, the line parallel to $AO$ passing through $D$ and the altitude from $A$ are concurrent, then the point of concurrence is the orthocenter of the triangle $ABC$. Proposed by Petar NiziƩ-Nikolac