IMO 1976 LL GBR15

Let ABC and A′B′C′ be any two coplanar triangles. Let L be

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IMO 1976 LL GBR15

Origin: GBR

Problem

Let ABC and A′B′C′ be any two coplanar triangles. Let L be a point such that AL\parallelBC, A′L\parallelB′C′, and M, N similarly defined. The line BC meets B′C′ at P, and similarly defined are Q and R. Prove that PL, QM, RN are concurrent.