Maria Flavia Mammana, Biagio Micale, and Mario Pennisi, Orthic quadrilaterals of a convex quadrilateral,
Forum Geometricorum, 10 (2010) 79--91.

Abstract:  We introduce the orthic quadrilaterals of a convex quadrilateral, based on the notion of valtitudes. These  orthic quadrilaterals have properties analogous to those of the orthic triangle  of a triangle.

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