Since we do not know which of the opposite sides is longer, we put absolute values
around the subtractions in the denominator to cover all cases. The area K of the cyclic quadrilateral is given by the well known Brahmagupta formula .
The formula for the area of the diagonal point triangle does not apply when two
opposite sides are congruent. Then the other two sides are parallel, so the cyclic
quadrilateral is an isosceles trapezoid