Prove $\cot \left( x + \frac { \pi } { 4 } \right) = \frac { 1 - \tan x } { \tan x + 1 }$

question 109

Essay

Prove $\cot \left( x + \frac { \pi } { 4 } \right) = \frac { 1 - \tan x } { \tan x + 1 }$ .