Given is a triangle and a circle with center that touches , and meets at , . The line through perpendicular to meets the line through parallel to at . Show that the circumcircles of and are tangent to each other.
Solution
Let be the midpoint of the major arc , be such that , be the intersection of the circle with diameter and and let touch , at , . We claim the two circles touch at .
Firstly, observe that , so
hence , , are collinear. Let . By spiral similarity and angle bisector theorem we have , so by converse of Menelaus theorem for we obtain that , , are concurrent at . Thus, by power of point at , we obtain that , so is cyclic.
Finally, observe that the center of lies on , so touches at . Hence, by shooting lemma, since is the midpoint of the minor arc , we obtain that touches at and we are done.
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