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IOM 2020 Problem 6

    In convex pentagon ABCDEABCDE points A1A_1 , B1B_1 , C1C_1 , D1D_1 , E1E_1 are intersections of pairs of diagonals (BD,CE)(BD, CE) , (CE,DA)(CE, DA) , (DA,EB)(DA, EB) , (EB,AC)(EB, AC) and (AC,BD)(AC, BD) respectively. Prove that if four of quadrilaterals AB1A1BAB_{1}A_{1}B , BC1B1CBC_{1}B_{1}C , CD1C1DCD_{1}C_{1}D , DE1D1EDE_{1}D_{1}E and EA1E1AEA_{1}E_{1}A are cyclic then the fifth one is also cyclic.

    Dataset text AI-MO AoPS. Quoted with source.

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