ISL 1980 Problem 10

Two circles $C_{1}$ and $C_{2}$ are (externally or internally) tangent at a point $P$ . The straight line $D$ is tangent at $A$ to one of the circles and cuts the other circle at the points $B$ and $C$ . Prove that the straight line $PA$ is an interior or exterior bisector of the angle $\angle BPC$ .