Descartes' Circle Formula
(based on wording of ARML 2010 Power)
Descartes' Circle Formula is a relation held between four mutually tangent circles.
Some notation: when discussing mutually tangent circles (or arcs), it is convenient to refer to the curvature of a circle rather than its radius. We define curvature as follows. Suppose that circle A of radius 
is externally tangent to circle B of radius 
. Then the curvatures of the circles are simply the reciprocals of their radii, 
and 
.
If circle A is internally tangent to circle B, however, a the curvature of circle A is still , while the curvature of circle B is 
, the opposite of the reciprocal of its radius.
[asy] draw(Circle(origin,2)); [/asy]
