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.