Online Geometry Problem 875: Congruent Circles, Intersecting Circles,
Secant, Center, Angle Trisection, Triple Angle, Triangle. Level: High School,
College,
Mathematics Education

The figure below shows two congruent circles
of center A and B intersecting at C and D. AC and AB extended intersect
circle B at E and F, respectively. Prove that angle EBF is three times
angle BAC,