$\"\"$

\

The trigonometric identity is $\"\"$ .

\

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

\

$\"\"$

\

Quotient functions : $\"\"$ and $\"\"$ .

\

$\"\"$

\

$\"\"$

\

Pythagorean identity: $\"\"$ .

\

$\"\"$

\

= Right hand side of the identity.

\

$\"\"$

\

$\"\"$ .