This is a special case of Brahmagupta's formula. It can also be derived directly from the trigonometric formula for the area of a tangential quadrilateral. Note that the converse does not hold: Some quadrilaterals that are not bicentric also have area K = a b c d . {\displaystyle \displaystyle K={\sqrt {abcd}}.} [12] One example of such a quadrilateral is a non-square rectangle.