Verify That Each Equation Is an Identity $$\cos ^ { 4 } x = \frac { 1 } { 8 } ( 3 + 4 \cos ( 2 x ) + \cos ( 4 x ) )$$

Verify that each equation is an identity.
- $$\cos ^ { 4 } x = \frac { 1 } { 8 } ( 3 + 4 \cos ( 2 x ) + \cos ( 4 x ) )$$