$\"\"$

The identity is $\"\"$

To take the left hand side expression for prove that right hand side expression.

$\"\"$

$\"\"$ (Add: $\"\"$ )

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

$\"\"$ (Apply Pythagorean identity: $\"\"$ )

$\"\"$

$\"\"$ (Cancel common terms) \ \

$\"\"$ (Apply reciprocal identity: $\"\"$ ) \ \ $\"\"$

$\"\"$ is an identity.