Let be the circumcircle of the triangle . Two circles , are tangent to each of the circle and the rays , , with interior to , exterior to . The common tangent of , and the common tangent of , intersect at the point . Let be the midpoint of the arc (not containing the point ) on the circle , and the segment be a diameter of . Prove that , , and are collinear.
Solutions — 2
Solution 1
(Solution 1): Let the point of tangency of , be , and the point of tangency of , be ; the original problem is equivalent to being a harmonic quadrilateral. Consider the transformation "inversion centered at with power , followed by reflection across the angle bisector of "; then , , remain fixed, is transformed into the -excircle (let its center be ), and is transformed into the incircle (let its center be ). Let , , be the feet of the perpendiculars from , , respectively to , and let be the intersection of the angle bisector of with ; then is transformed to , is transformed to , is transformed to , and is transformed to ; the original problem is equivalent to being a harmonic range, which is again equivalent to being a harmonic range. Since , the original proposition holds.

Let meet at , meet at , let be tangent to at , be tangent to at , be tangent to at , be tangent to at . By Mannheim's theorem, , , are collinear, are collinear, and , . By the Incenter-Excenter Lemma, , hence , from which it follows that is indeed the radical axis of , . This completes the proof.

Solution 2
It suffices to prove that is the radical axis of , ; then is the radical center of the three circles, and the original proposition is thereby proved.
Let meet at , meet at , let be tangent to at , be tangent to at , be tangent to at , be tangent to at . By Mannheim's theorem, , , are collinear, are collinear, and , . By the Incenter-Excenter Lemma, , hence , from which it follows that is indeed the radical axis of , . This completes the proof.
