Online Geometry Problem 799: Parallelogram, Angle
Bisector, Parallel, Triangle, Circumcircle, Circumcenter. Level: High School,
SAT Prep, College,
Mathematics Education

The figure below shows a parallelogram ABCD,
AE is the bisector of angle BAD. If AE and DC meet at F, prove that the
circumcenter Q of triangle CEF lies on the circumcircle of triangle BCD.