olymmath

IMO 2021 Problem 3

  • Day 1

Let DD be an interior point of the acute triangle ABCABC with AB>ACAB > AC so that ∠DAB=∠CAD.\angle DAB = \angle CAD. The point EE on the segment ACAC satisfies ∠ADE=∠BCD,\angle ADE =\angle BCD, the point FF on the segment ABAB satisfies ∠FDA=∠DBC,\angle FDA =\angle DBC, and the point XX on the line ACAC satisfies CX=BX.CX = BX. Let O1O_1 and O2O_2 be the circumcenters of the triangles ADCADC and EXD,EXD, respectively. Prove that the lines BC,EF,BC, EF, and O1O2O_1O_2 are concurrent.

Official text Transcription from AI-MO AoPS, checked word for word against the official paper. Quoted with source.

Official text: IMO 2021 official English paper, page 1.

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