Prove the Identity $$\sin 4 x = ( 4 \sin x \cos x ) \left( 1 - 2 \sin ^ { 2 } x \right)$$

Essay

Prove the identity.
- $$\sin 4 x = ( 4 \sin x \cos x ) \left( 1 - 2 \sin ^ { 2 } x \right)$$