ISL 1996 G1
Let be a triangle, and its orthocenter. Let be a point on the circumcircle of triangle (distinct from the vertices , , ), and let be the foot of the altitude of triangle from the vertex . Let the parallel to the line through the point meet the parallel to the line through the point at a point . Let the parallel to the line through the point meet the parallel to the line through the point at a point . The lines and intersect at some point . Prove that the lines and are parallel.
Dataset text AI-MO AoPS. Quoted with source.
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