In triangle ABC the points N and M are on the side such that . The points P and Q are on the angle bisector such that A, P, and Q are all on the same side with respect to and . Let and meet at and and meet at , prove that the common tangents of the circumcircles of and intersect on the circumcircle of .
Solution
We shall prove the following lemma.

Lemma 1. *The points , , , and are on a circle centered at .*
Proof. An easy angle chasing shows that and are indeed cyclic.
Thus,
Yielding is cyclic. Then,
and
And
Now, we need the following lemma;
Lemma 2. *The quadrilateral is cyclic*
Proof. According to the above-mentioned lemma; . This completes our proof.

Thus the radical axis of , , and are concurrent. Let be the foot of ex-angle bisector of , and assume and meet at . Since and meet at , it follows that is on , indeed . Yielding . That is;
Yielding to the fact that is the intersection of the corresponding arcs of and . Therefore, . Whence, . This implies that . According to the first lemma, and is on the perpendicular bisector of . Let be the intersection point of the common tangents of and , taking into account that is the center of the homothety of these circles, it follows that . Therefore, , and would be collinear and is on the perpendicular bisector of . Hence, is the intersection of the corresponding arc of the and its perpendicular bisector. Since such a point would be unique, we find that . We are done. ■
