Step 1:

The identity is $\"image\"$

Consider the left hand part $\"image\"$

Recall the trigonometric identity $\"image\"$

$\"image\"$

$\"image\"$ (Since $\"image\"$ )

$\"image\"$

For even multiples of $\"image\"$ cosine function value is 1 and for odd multiples of $\"image\"$ cosine function value is -1.

$\"image\"$

Solution:

The identity $\"image\"$ is verified.