$\"\"$

The identity is $\"\"$

Left hand side identity $\"\"$

Recall the trigonometric identity $\"\"$ .

$\"image\"$ \ \

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

$\"image\"$

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

$\"\"$ .

$\"\"$

The identity $\"\"$ is verified.