$\"\"$

\

The cofunction identity is $\"\"$ .

\

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

\

Apply reciprocal identity : $\"\"$ .

\

$\"\"$

\

Apply quotient identity : $\"\"$ .

\

$\"\"$

\

Apply difference identities : $\"\"$

\

$\"\"$ .

\

$\"\"$

\

$\"\"$

\

= Right hand side identity.

\

$\"\"$

\

$\"\"$ .