A relation for the side lengths of a cyclic one of these is given by Ptolemy's theorem.
The area of one of these figures can be found with Bretschneider's formula, which reduces to Brahmaguptra's formula for special cases. In general, their area is "one half the length of the cross product of the diagonals," and Van Aubel's Theorem shows that the lines connecting the centers of squares constructed on one's sides are perpendicular and equal. A relation for the side lengths of a cyclic one of these is given by Ptolemy's theorem. Specific examples of them include kites, trapezoids, and parallelograms. For 10 points, name this figure which denotes any object with four sides.
The area of one of these figures can be found with Bretschneider's formula, which reduces to Brahmaguptra's formula for special cases.
{quadrilateral} [do not accept or prompt on any {specific type} of {quadrilateral}]