Problem

Source: MOP 2005 Homework - Black Group #20

Tags: geometry, circumcircle, AMC, USA(J)MO, USAMO, geometric transformation, geometry unsolved



Given a convex quadrilateral $ABCD$. The points $P$ and $Q$ are the midpoints of the diagonals $AC$ and $BD$ respectively. The line $PQ$ intersects the lines $AB$ and $CD$ at $N$ and $M$ respectively. Prove that the circumcircles of triangles $NAP$, $NBQ$, $MQD$, and $MPC$ have a common point.