Let be the midpoint of side in triangle . is a point on segment such that . The circumcircles of triangles and , and , intersect for the second time at point . Let be the midpoint of arc of . Lines and intersect at point . Prove that line passes through the midpoint of segment .
Solution
Let the bisector of intersect for the second time at point . Note that is the midpoint of arc (that does not contain ) of . Also, it is easy to see that line is the perpendicular bisector of segment . Thus, , i.e. points and lie on a circle centered at with radius . Since , then lines and are tangent to this circle. Therefore, line contains the symmedian of triangle corresponding to vertex .
Let line intersect for the second time at point . From the properties of symmedian it follows that
Also, the following equalities hold:
From (1) and (2) it follows that . The latter equality gives
Let line intersect line at point . Then, from (2) it follows that , or that is a midsegment of triangle , i.e. is the midpoint of segment . Now it is enough to note that if line bisects segment , then it also bisects segment , since (3) holds.