IMO Shortlist 2017 problem G5


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3. listopada 2019.
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Let ABCC_1B_1A_1 be a convex hexagon such that AB=BC, and suppose that the line segments AA_1, BB_1, and CC_1 have the same perpendicular bisector. Let the diagonals AC_1 and A_1C meet at D, and denote by \omega the circle ABC. Let \omega intersect the circle A_1BC_1 again at E \neq B. Prove that the lines BB_1 and DE intersect on \omega.

Izvor: https://www.imo-official.org/problems/IMO2017SL.pdf