olymmath

IMO 2025 Problem 2

  • Day 1
  • Geometry
  • Proof

Let Ω\Omega and Γ\Gamma be circles with centres MM and NN, respectively, such that the radius of Ω\Omega is less than the radius of Γ\Gamma. Suppose circles Ω\Omega and Γ\Gamma intersect at two distinct points AA and BB. Line MNMN intersects Ω\Omega at CC and Γ\Gamma at DD, such that points CC, MM, NN and DD lie on the line in that order. Let PP be the circumcentre of triangle ACDACD. Line APAP intersects Ω\Omega again at E≠AE \neq A. Line APAP intersects Γ\Gamma again at F≠AF \neq A. Let HH be the orthocentre of triangle PMNPMN.

Prove that the line through HH parallel to APAP is tangent to the circumcircle of triangle BEFBEF.

(The orthocentre of a triangle is the point of intersection of its altitudes.)

Official text IMO 2025 official English paper, page 1. Quoted with source.

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