Write the Word or Phrase That Best Completes Each Statement $$\cos ^ { 4 } x = \frac { 1 } { 8 } ( 3 + 4 \cos 2 x + \cos 4 x )$$

Essay

Write the word or phrase that best completes each statement or answers the question.
- $$\cos ^ { 4 } x = \frac { 1 } { 8 } ( 3 + 4 \cos 2 x + \cos 4 x )$$