Online Geometry
Problem 475: Tangent Circles, Secant, Tangent, Parallel

The figure shows the circles C_{1}
and C_{2} internally tangent at A. Line ABC is a secant,
line L_{1} is tangent to C_{1} at C and line L_{2}
is tangent to C_{2} at B. Prove that lines L_{1}
and L_{2} are parallel.