Problem 737: Cyclic Quadrilateral, Circumscribed Circle,
Level: High School, Honors Geometry, College,
The figure shows a cyclic quadrilateral ABCD
where AD is the diameter of the circumscribed circle. AB and DC extended
meet at E. Tangents at B and C meet at F. Prove that EF is perpendicular
to AD. This entry contributed by Ajit Athle.