Cotangent of Sum of Three Angles

Theorem

 * $\map \cot {A + B + C} = \dfrac {\cot A + \cot B + \cot C - \cot A \cot B \cot C} {1 - \cot B \cot C - \cot C \cot A - \cot A \cot B}$

Also see

 * Tangent of Sum of Three Angles