Write the Word or Phrase That Best Completes Each Statement $$\sin \left( \frac { \pi } { 4 } + x \right) = \sqrt { 2 } ( \cos x + \sin x )$$

Essay

Write the word or phrase that best completes each statement or answers the question.
Establish the identity.
- $$\sin \left( \frac { \pi } { 4 } + x \right) = \sqrt { 2 } ( \cos x + \sin x )$$