olymmath

IGO 2021 I3

  • Intermediate

Given a convex quadrilateral ABCDABCD with AB=BCAB = BC and ∠ABD=∠BCD=90\angle ABD = \angle BCD = 90 .Let point EE be the intersection of diagonals ACAC and BDBD . Point FF lies on the side ADAD such that AFFD=CEEA\frac{AF}{F D}=\frac{CE}{EA} .. Circle ω\omega with diameter DFDF and the circumcircle of triangle ABFABF intersect for the second time at point KK . Point LL is the second intersection of EFEF and ω\omega . Prove that the line KLKL passes through the midpoint of CECE . Proposed by Mahdi Etesamifard and Amir Parsa Hosseini - Iran

Dataset text AI-MO AoPS. Quoted with source.

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