Nicusor Minculete, Characterizations of a tangential quadrilateral,
Forum Geometricorum, 9 (2009) 113--118.

Abstract: In this paper we will present several relations about the tangential quadrilaterals;  among these, we have that the quadrilateral ABCD is tangential if and only if the  following equality 1/d(O,AB) + 1/d(O,CD) = 1/d(O,BC) + 1/d(O,DA) holds, where O  is the point where the diagonals of convex  quadrilateral ABCD meet.  This is equivalent  to Wu's Theorem.

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