Problem 724: Circumscribed Quadrilateral, Circle, Angle, Measure.
High School, College

In the diagram, ABCD is a
circumscribed quadrilateral to circle O. E is a point on OA and
F is a point on OC. If the measure of angle EBF is half the
measure of angle ABC, prove that the measure of angle EDF is
half the measure of angle ADC.