Write the Word or Phrase That Best Completes Each Statement $$\tan \left( x - \frac { \pi } { 4 } \right) = \frac { \tan x - 1 } { 1 + \tan x }$$

Essay

Write the word or phrase that best completes each statement or answers the question.
Establish the identity.
- $$\tan \left( x - \frac { \pi } { 4 } \right) = \frac { \tan x - 1 } { 1 + \tan x }$$