Proof. If A and B are the centers of the circles (Fig. 1), then, obviously, f can
be represented as scaling by factor P B/P A with respect to P followed by the
rotation through ∠AP B about P. Then all the triangles P XY are directly
similar to triangle P AB, and therefore, the (signed) angle P XY is constant mod
π; hence, by the Inscribed Angle Theorem, the second intersection point of XY
and a is fixed. But for X = f
−1
(Q) this point is Q.