$\"\"$

Let $\"\"$ , $\"\"$ and $\"\"$ .

$\"\"$ (Secant theorem)

$\"\"$ (Substitute $\"\"$ , $\"\"$ and $\"\"$ )

$\"\"$

$\"\"$ (Cancel common terms)

$\"\"$ (Subtract: $\"\"$ ) $\"\"$ $\"\"$ (Multiply each side by 2)

$\"\"$ (Simplify)

$\"\"$ (Divide each side by 8)

$\"\"$ (Cancel common terms)

$\"\"$ (Divide: $\"\"$ ) $\"\"$

The value of $\"\"$ .