# Thread: proving for tan question

1. ## proving for tan question

Hey need some help with this one

Prove that $tan (\theta +\phi ) = (tan \theta + tan \phi) \div ( 1 - tan \theta . tan \phi )$

Cheers

2. $\sin ( \theta + \phi ) = \sin \theta \cos \phi + \cos \theta \sin \phi$

$\cos (\theta + \phi ) = \cos \theta \cos \phi - \sin \theta \sin \phi$

$\tan (\theta + \phi ) = \frac{\sin \theta \cos \phi + \cos \theta \sin \phi }{\cos \theta \cos \phi - \sin \theta \sin \phi}$

divide the denominator and nominator by $\cos \phi \cos \theta$

you will get the identity

3. thanks for the reply Amer