Problem

Source: 2010 Oral Moscow Geometry Olympiad grades 10-11 p6

Tags: geometry, concurrency, concurrent, Circumcenter, parallel



In a triangle $ABC, O$ is the center of the circumscribed circle. Line $a$ passes through the midpoint of the altitude of the triangle from the vertex $A$ and is parallel to $OA$. Similarly, the straight lines $b$ and $c$ are defined. Prove that these three lines intersect at one point.