olymmath

ISL 2018 G4

    A point TT is chosen inside a triangle ABCA B C. Let A1,B1A_{1}, B_{1}, and C1C_{1} be the reflections of TT in BC,CAB C, C A, and ABA B, respectively. Let Ω\Omega be the circumcircle of the triangle A1B1C1A_{1} B_{1} C_{1}. The lines A1T,B1TA_{1} T, B_{1} T, and C1TC_{1} T meet Ω\Omega again at A2,B2A_{2}, B_{2}, and C2C_{2}, respectively. Prove that the lines AA2,BB2A A_{2}, B B_{2}, and CC2C C_{2} are concurrent on Ω\Omega. (Mongolia)

    Dataset text NuminaMath olympiads. Quoted with source.

    Official text: IMO 2018 Shortlist booklet, page 8.

    Solutions

    Official solution Solution 1 by the IMO Problem Selection Committee

    The terms of the source let olymmath link to this solution but not copy it. Read the solution (IMO official site).

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