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MMO2025 Round 4

Mongolia 2025 geometry

Problem

Let be the incenter of triangle , which is inscribed in the circle centered at . Suppose the line intersects again at point . Let be the reflection of over the line . Suppose that the line intersects again at point , and the line intersects again at point . Prove that the lines and are parallel. (Batzorig Undrakh)
Solution
Note that since , it follows that . Also, since , we have Let us compute the power of point with respect to the circle . Since , we get

, $OI \parallel BE$.

Techniques

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