Solve the Problem $$\left( \cos \frac { x } { 2 } - \sin \frac { x } { 2 } \right) ^ { 2 } = 1 - \sin x$$

Solve the problem.

- $$\left( \cos \frac { x } { 2 } - \sin \frac { x } { 2 } \right) ^ { 2 } = 1 - \sin x$$