olymmath

IGO 2016 A4

  • Advanced

In a convex quadrilateral ABCDABCD , the lines ABAB and CDCD meet at point EE and the lines ADAD and BCBC meet at point FF . Let PP be the intersection point of diagonals ACAC and BDBD . Suppose that ω1\omega_1 is a circle passing through DD and tangent to ACAC at PP . Also suppose that ω2\omega_2 is a circle passing through CC and tangent to BDBD at PP . Let XX be the intersection point of ω1\omega_1 and ADAD , and YY be the intersection point of ω2\omega_2 and BCBC . Suppose that the circles ω1\omega_1 and ω2\omega_2 intersect each other in QQ for the second time. Prove that the perpendicular from PP to the line EFEF passes through the circumcenter of triangle XQYXQY . Proposed by Iman Maghsoudi

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