Hyperbolic Tangent of Sum

Theorem

 * $\tanh \left({a + b}\right) = \dfrac {\tanh a + \tanh b} {1 + \tanh a \tanh b}$

where $\tanh$ denotes the hyperbolic tangent.

Also see

 * Hyperbolic Sine of Sum
 * Hyperbolic Cosine of Sum
 * Hyperbolic Cotangent of Sum