Tangential quadrilateral ( the sum of the opposite sides)

Description

In Euclidean geometry, a tangential quadrilateral (sometimes just tangent quadrilateral) or circumscribed quadrilateral is a convex quadrilateral whose sides are all tangent to a single circle within the quadrilateral. This circle is called the incircle of the quadrilateral or its inscribed circle, its center is the incenter and its radius is called the inradius. In a tangential quadrilateral, the four angle bisectors meet at the center of the incircle. The two pairs of opposite sides in a tangential quadrilateral add up to the same total length. Conversely a convex quadrilateral in which the two pairs of opposite sides add up to the same total length must be tangential.