If a Quadrilateral is inscribed in a circle (i.e., for a cyclic quadrilateral), the sum of the products of the two pairs
of opposite sides equals the product of the diagonals

This fact can be used to derive the Trigonometry addition formulas.

**References**

Coxeter, H. S. M. and Greitzer, S. L. *Geometry Revisited.* Washington, DC: Math. Assoc. Amer., pp. 42-43, 1967.

© 1996-9

1999-05-26