olymmath

IMO 2026 Problem 2

  • Day 1
  • Geometry
  • Proof

Let ABCABC be a triangle and let points MM and NN be the midpoints of sides ABAB and ACAC, respectively. Let points KK and LL be chosen strictly inside triangles BMCBMC and BNCBNC, respectively, such that KK lies strictly inside triangle ABLABL and LL lies strictly inside triangle AKCAKC. Suppose that

∠KBA=∠ACL,∠LBK=∠LNC,and∠LCK=∠BMK.\angle KBA = \angle ACL, \quad \angle LBK = \angle LNC, \quad \text{and} \quad \angle LCK = \angle BMK.

Let OO be the circumcentre of triangle AKLAKL. Prove that OM=ONOM = ON.

Official text IMO 2026 official English paper, page 1. Quoted with source.

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