olymmath

ISL 2021 G7

    A point DD is chosen inside an acute-angled triangle ABCA B C with AB>ACA B>A C so that ∠BAD=∠DAC\angle B A D=\angle D A C. A point EE is constructed on the segment ACA C so that ∠ADE=∠DCB\angle A D E=\angle D C B. Similarly, a point FF is constructed on the segment ABA B so that ∠ADF=∠DBC\angle A D F=\angle D B C. A point XX is chosen on the line ACA C so that CX=BXC X=B X. Let O1O_{1} and O2O_{2} be the circumcentres of the triangles ADCA D C and DXED X E. Prove that the lines BC,EFB C, E F, and O1O2O_{1} O_{2} are concurrent.

    Dataset text NuminaMath olympiads. Quoted with source.

    Official text: IMO 2021 Shortlist booklet, page 9.

    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).

    Locator isl/2021/g7 · Tag KSY7CSS4P7 · Edit this page on GitHub · Keys: j next, k previous