Atul Dixit and Darij Grinberg, Orthopoles and the Pappus
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.