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55th IMO Team Selection Test

Bulgaria geometry

Problem

Given is a with incircle touching the sides and in points and respectively. Let denote with the center of the excircle at side in and with – the second intersection point of the circumcircles of and . Prove that the circumcircle of touches .

problem
Solution
Let be the tangent point of to . We will use the standard notations about the angles of . We have and Then i.e. and hence , .

i.e. is the midpoint of . But the homothety sends to . Therefore these circles are tangent at point .

Techniques

Triangle centers: centroid, incenter, circumcenter, orthocenter, Euler line, nine-point circleTangentsHomothetyAngle chasing