Online Geometry Problem 701: Intersecting Circles, Diameter,
Perpendicular, Angles, Congruence.
Level: High School, Honors Geometry, College,
Mathematics Education

The figure shows the intersecting circles O
and O' at A and B. AB is the diameter of circle O', CD is perpendicular
to AB and EDF is perpendicular to OD. Prove that angles ACF and BCE are
congruent.