$\"\"$

\

The trigonometric identity is $\"fraction$ .

\

Consider left hand side $\"\"$ .

\

$\"\"$ $\"\"$

\

$\"\"$

\

Apply Pythagorean identity: $\"\"$ .

\

$\"\"$

\

$\"\"$

\

= Right hand side.

\

$\"fraction$ . $\"\"$

\

$\"fraction$ .