A beautiful elementary proof of this was given by Salazar (see [8], and quoted at
[1]). According to [9, p.292], this is also a sufficient condition for the existence
of a bicentric quadrilateral. Now if there for two such circles exists one bicentric
quadrilateral, then according to Poncelet’s closure theorem there exists infinitely
many; any point on the circumcircle can be a vertex for one of these bicentric
quadrilaterals [11]. That is the configuration we shall study in this note. We derive
a formula for the area of a bicentric quadrilateral in terms of the inradius, the
circumradius and the angle between the diagonals, conclude for which quadrilateral
the area has its maximum value in terms of the two radii, and show how to construct
that maximal quadrilateral.