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Baltic Way shortlist

Baltic Way geometry

Problem

Let be an acute triangle, its orthocentre, and the midpoint of . Furthermore, let and be the circle with diameter and the circle with center that touches the circumcircle of triangle interiorly, respectively. Prove that and are touching circles.
Solution
Let be the midpoint of (and of ), and let be the image of with respect to reflection about . Then lies on the circumcircle of , opposite to . As and are parallel, by the Intercept Theorem, we have . Hence, , i.e., is a parallelogram. Let and be the radii of and , respectively, and let be the radius of 's circumcircle. Then and, hence, . This means that the distance between the midpoints of and is the sum of their radii. Consequently, and touch each other.

Techniques

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