The tangential quadrilateral is a convex quadrilateral in euclidean geometry,whose all sides are tangent to a single circle within the quadrilateral. In other word a quadrilateral which has an in-circle. This is also called as circumscribed quadrilateral.
Let a convex quadrilateral ABCD with sides a, b, c, d, then the area of a Tangential quadrilateral
a + c = b + d :
Since opposite angles are supplementary angles.
Below are examples on cyclic quadrilateral: