Excircle
Given a Triangle, extend two nonadjacent sides. The Circle tangent to these two lines and to the otherside of the Triangle is called an Escribed Circle, or excircle. The Center 
of the excircle iscalled the Excenter and lies on the external Angle Bisector of the opposite Angle. EveryTriangle has three excircles, and the Trilinear Coordinates of the Excenters are
, 
, and 
. The Radius 
of the excircle 
is called its Exradius.
Given a Triangle with Inradius 
, let 
be the Altitudes of the excircles, and their Radii (the Exradii). Then
(Johnson 1929, p. 189).See also Excenter, Excenter-Excenter Circle, Excentral Triangle,Feuerbach's Theorem, Nagel Point, Triangle Transformation Principle
References
Coxeter, H. S. M. and Greitzer, S. L. Geometry Revisited. Washington, DC: Math. Assoc. Amer., pp. 11-13, 1967. Johnson, R. A. Modern Geometry: An Elementary Treatise on the Geometry of the Triangle and the Circle. Boston, MA: Houghton Mifflin, pp. 176-177 and 182-194, 1929.
- Cross-Correlation Coefficient
- Cross-Correlation Theorem
- Cross Curve
- Crossed Ladders Problem
- Crossed Trough
- Cross Fractal
- Crossing Number (Graph)
- Crossing Number (Link)
- Cross of Lorraine
- Cross Polytope
- Cross Product
- Cross-Ratio
- Cross-Section
- Crout's Method
- Crowd
- Crown
- Crucial Point
- Cruciform
- Crunode
- Cryptarithm
- Cryptarithmetic
- Cryptography
- Crystallography Restriction
- Csch
- Császár Polyhedron