olymmath

IGO 2021 I5

  • Intermediate

Consider a convex pentagon ABCDEABCDE and a variable point XX on its side CDCD . Suppose that points K,LK, L lie on the segment AXAX such that AB=BKAB = BK and AE=ELAE = EL and that the circumcircles of triangles CXKCXK and DXLDXL intersect for the second time at YY . As XX varies, prove that all such lines XYXY pass through a fixed point, or they are all parallel. Proposed by Josef Tkadlec - Czech Republic

Dataset text AI-MO AoPS. Quoted with source.

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