Atul Dixit and Darij Grinberg,  Orthopoles and the Pappus Theorem,
Forum Geometricorum, 4 (2004) 53--59.

Abstract: If the vertices of a triangle are projected onto a given line,  the perpendiculars from the projections to the corresponding sidelines of the triangle intersect at one point, the orthopole of the line with respect to the triangle. We prove several theorems on orthopoles using the Pappus theorem, a fundamental result of projective geometry.

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