Paris Pamfilos, On some elementary properties of quadrilaterals,
Forum Geometricorum, 17 (2017) 473--482.

Abstract. We study some elementary properties of convex quadrilaterals related to the idea of common harmonics of two pairs of points and detecting two similar parallelograms with diagonals parallel to those of the quadrilateral and such that the area of the quadrilateral is the mean geometric of their areas. We apply these results to the study of variation of area of quadrilaterals, whose angles remain fixed in measure.

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