$\"\"$

The identity is $\"\"$ .

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

$\"\"$

(Multiply numerator and denominator by $\"\"$ )

$\"\"$

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

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

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

$\"\"$ is an identity.