IMO 1966 LL POL15
Points A, B, C, D lie on a circle such that AB is a diameter and
IMO 1966 LL POL15
Origin: POL
Problem
Points A, B, C, D lie on a circle such that AB is a diameter and CD is not. If the tangents at C and D meet at P while AC and BD meet at Q, show that PQ is perpendicular to AB.