Two circles are given for which there is a family of quadrilaterals circumscribed around the first circle and inscribed in the second. Let's denote by a,b,c and d the consecutive lengths of the sides of one of these quadrilaterals. Prove that the sum ca+ac+db+bddoes not depend on the choice of the quadrilateral.Proposed by I. Weinstein geometrybicentric quadrilateralKvant