# Descartes' theorem ( externally tangent circle to three given kissing circles)

## Description

In geometry, Descartes’ theorem states that for every four kissing, or mutually tangent, circles, the radii of the circles satisfy a certain quadratic equation. By solving this equation, one can construct a fourth circle tangent to three given, mutually tangent circles. In mathematics, curvature is the amount by which a geometric object deviates from being flat, or straight in the case of a line. Descartes’ theorem is most easily stated in terms of the circles’ curvatures. The curvature (or bend) of a circle is defined as k = ±1/r, where r is its radius. The larger a circle, the smaller is the magnitude of its curvature, and vice versa. If four circles are tangent to each other at six distinct points, and the circles have curvatures k1,k2,k3,k4 and trying to find the radius of the fourth circle that is externally tangent to three given kissing circles, Descartes’ theorem is giving the solution.

Related formulas## Variables

k_{4} | Curvature (or bend) of the externally tangent circle (m^{-1}) |

k_{1} | Curvature of the circle having radius r1 (m^{-1}) |

k_{2} | Curvature of the circle having radius r2 (m^{-1}) |

k_{3} | Curvature of the circle having radius r3 (m^{-1}) |