olymmath

ISL 2019 G7

    The incircle ω\omega of acute-angled scalene triangle ABCA B C has centre II and meets sides BCB C, CAC A, and ABA B at D,ED, E, and FF, respectively. The line through DD perpendicular to EFE F meets ω\omega again at RR. Line ARA R meets ω\omega again at PP. The circumcircles of triangles PCEP C E and PBFP B F meet again at Q≠PQ \neq P. Prove that lines DID I and PQP Q meet on the external bisector of angle BACB A C. (India) Common remarks. Throughout the solution, ∠(a,b)\angle(a, b) denotes the directed angle between lines aa and bb, measured modulo π\pi.

    Dataset text NuminaMath olympiads. Quoted with source.

    Official text: IMO 2019 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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