$\"\"$

\

The trigonometric identity is $\"\"$ .

\

Consider left hand side identity :

\

$\"\"$

\

$\"\"$

\

Apply pythagorean identity : $\"\"$ .

\

$\"\"$

\

$\"\"$

\

$\"\"$

\

= Right hand side identity.

\

$\"\"$

\

Thus, $\"\"$ .