olymmath

ISL 2023 G8

    Let ABCA B C be an equilateral triangle. Points A1,B1,C1A_{1}, B_{1}, C_{1} lie inside triangle ABCA B C such that triangle A1B1C1A_{1} B_{1} C_{1} is scalene, BA1=A1C,CB1=B1A,AC1=C1BB A_{1}=A_{1} C, C B_{1}=B_{1} A, A C_{1}=C_{1} B and ∠BA1C+∠CB1A+∠AC1B=480∘.\angle B A_{1} C+\angle C B_{1} A+\angle A C_{1} B=480^{\circ} . Lines BC1B C_{1} and CB1C B_{1} intersect at A2A_{2}; lines CA1C A_{1} and AC1A C_{1} intersect at B2B_{2}; and lines AB1A B_{1} and BA1B A_{1} intersect at C2C_{2}. Prove that the circumcircles of triangles AA1A2,BB1B2,CC1C2A A_{1} A_{2}, B B_{1} B_{2}, C C_{1} C_{2} have two common points. (U.S.A.)

    Dataset text NuminaMath olympiads. Quoted with source.

    Official text: IMO 2023 Shortlist booklet, page 10.

    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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