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KöMaL problem A, 2024

Geometry Difficulty 9.0 IMO level Prove it Hungary

Let ABCDABCD be a convex cyclic quadrilateral satisfying AB⋅CD=AD⋅BCAB\cdot CD=AD\cdot BC. Let the inscribed circle ω\omega of triangle ABCABC be tangent to sides BCBC, CACA and ABAB at points A′A', B′B' and C′C', respectively. Let point KK be the intersection of line IDID and the nine-point-circle of triangle A′B′C′A'B'C' that is inside line segment IDID. Let SS denote the centroid of triangle A′B′C′A'B'C'. Prove that lines SKSK and BB′BB' intersect each other on circle ω\omega.

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