Prove the identities

1. $$\frac{1}{\tan u}=\frac{\cos u}{\sin u}$$
2. $$\tan \theta +(\tan \theta {)}^{-1}=\frac{1}{\sin \theta \cos \theta }$$
3. $$\sin \phi =\frac{\tan \phi }{\sqrt{1+{\tan }^2\phi }}\text{ and }\cos \phi =\frac{1}{\sqrt{1+{\tan }^2\phi }}$$