Problem 300: Tangents to a circle, Secants, Minor Arc.
Level: High School, College, SAT Prep.

In the figure, AB and AC are tangent to circle O at B and C, respectively, and D is a point on the minor arc BC. Lines BD and AC meet at E, and lines CD and AB meet at F. If BF = a, CE = b, and EF = x, prove that .