A Circumcircle through the Midpoint of a Triangle's Side
Let ABC be a triangle. Let the feet of the altitudes from A, B, and C be A', B' and C', respectively. Let the line through A' parallel to B'C' meet AC and AB at Q and R, respectively. Let the line B'C' meet BC at P. Then the circumcircle of PQR passes through the midpoint M of BC.