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37th Iranian Mathematical Olympiad

Iran geometry

Problem

Consider a triangle with circumcenter and incenter . The incircle touches sides , and at , and , respectively. Let be a point such that is tangent to circumcircle of and is tangent to circumcircle of . Prove that , and are concurrent.
Solution
Note that Which is equivalent to Using Ceva's theorem in triangle for point . Now, by the Ratio lemma, we have By Ceva's theorem in for point , we have So we have just to prove that Which is obviously true since .

Techniques

Triangle centers: centroid, incenter, circumcenter, orthocenter, Euler line, nine-point circleCeva's theoremTangentsTriangle trigonometryAngle chasing