Exradius
Excircle
The radius of an excircle. Let a triangle have exradius r_A (sometimes denoted rho_A), opposite side of length a and angle A, area Delta, and semiperimeter s. Then
(Johnson 1929, p. 189), where R is the circumradius. Let r be the inradius, then
(4)
(5)
(Casey 1888, p. 65) and
(6)
Some fascinating formulas due to Feuerbach are