Online Geometry Problem 737: Cyclic Quadrilateral, Circumscribed Circle, Diameter, Tangent, Perpendicular. Level: High School, Honors Geometry, College, Mathematics Education

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.