$\"\"$

The identity is $\"\"$ .

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

$\"\"$

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

$\"\"$ (Apply formula: $\"\"$ )

$\"\"$ (Cancel common terms)

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

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

$\"\"$ is an identity.