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.
- Pappus's Centroid Theorem
- Pappus's Harmonic Theorem
- Pappus's Hexagon Theorem
- Pappus's Theorem
- Parabiaugmented Dodecahedron
- Parabiaugmented Hexagonal Prism
- Parabiaugmented Truncated Dodecahedron
- Parabidiminished Rhombicosidodecahedron
- Parabigyrate Rhombicosidodecahedron
- Parabola
- Parabola Caustic
- Parabola Evolute
- Parabola Inverse Curve
- Parabola Involute
- Parabola Pedal Curve
- Parabolic Coordinates
- Parabolic Cyclide
- Parabolic Cylinder
- Parabolic Cylinder Function
- Parabolic Cylindrical Coordinates
- Parabolic Fixed Point
- Parabolic Geometry
- Parabolic Horn Cyclide
- Parabolic Partial Differential Equation
- Parabolic Point