## Precalculus (6th Edition) Blitzer

Identity for $\tan \left( \theta +\phi \right)$ is expressed as $\frac{\tan \theta +\tan \phi }{1-\tan \theta \tan \phi }$.
From the sum formula of tangents, the tangent of the sum of two angles, say A and B, is expressed as, $\tan \left( A+B \right)=\frac{\tan A+\tan B}{1-\tan A\tan B}$ Thus, for angles $\theta$ and $\phi$ , $\tan \left( \theta +\phi \right)=\frac{\tan \theta +\tan \phi }{1-\tan \theta \tan \phi }$