$\"\"$

\

The identity is $\"\"$ .

\

Consider left hand side identity : $\"\"$ .

\

Let $\"\"$ , then $\"\"$ .

\

$\"\"$

\

Apply sine power reducing identity : $\"\"$ .

\

$\"\"$

\

Substitute $\"\"$ in above expression.

\

$\"\"$

\

= Right hand side identity.

\

$\"\"$

\

$\"\"$ .