Ptolemy's Theorem
If a Quadrilateral is inscribed in a circle (i.e., for a cyclic quadrilateral), the sum of the products of the two pairsof opposite sides equals the product of the diagonals
This fact can be used to derive the Trigonometry addition formulas.See also Fuhrmann's Theorem, Ptolemy Inequality
References
Coxeter, H. S. M. and Greitzer, S. L. Geometry Revisited. Washington, DC: Math. Assoc. Amer., pp. 42-43, 1967.
- Parabolic Spindle Cyclide
- Parabolic Spiral
- Parabolic Umbilic Catastrophe
- Paraboloid
- Paraboloidal Coordinates
- Paraboloid Geodesic
- Paracompact Space
- Paracycle
- Paradox
- Paradromic Rings
- Paragyrate Diminished Rhombicosidodecahedron
- Parallel
- Parallel Axiom
- Parallel Class
- Parallel Curve
- Parallelepiped
- Parallelism
- Parallelizable
- Parallelogram
- Parallelogram Illusion
- Parallelogram Law
- Parallelohedron
- Parallelotope
- Parallel Postulate
- Parallel (Surface of Revolution)