Problem:
Let be a quadrilateral circumscribed about a circle with center . Let , and denote the circumcenters of , , , and . If , , and , what is the acute angle formed by the two lines passing through and ?
Problem:
Let be a quadrilateral circumscribed about a circle with center . Let , and denote the circumcenters of , , , and . If , , and , what is the acute angle formed by the two lines passing through and ?
Solution:
Answer:
Lemma: Given a triangle , let be the incenter, be the excenter opposite , and be the second intersection of with the circumcircle. Then is the center of the circle through , and .
Proof. First, note
Similarly . Therefore is cyclic. Now note that , and are collinear because they are all on the angle bisector of . Hence
(Note since , and are concyclic.) Hence . Similarly . Thus is the center of the circle passing through , and , and therefore as well.
Let and intersect at and and intersect at . We first show that , and are collinear.
Let and denote the intersections of with the circumcircles of triangles and . Since is the excenter of triangle , by the lemma is the circumcenter of ; since is incenter of triangle , by the lemma is the circumcenter of . Hence and . Thus, points , and are collinear, and similarly, we have , and are collinear.
Now and so considering quadrilateral , the angle between and is