$\"\"$

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

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$\"\"$ (Intersection of a secant and a tangent)

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$\"\"$ ( $\"\"$ )

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

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$\"\"$ (Multiply: $\"\"$ ) $\"\"$ $\"\"$ (Subtract 4 from each side)

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$\"\"$ (Simplify)

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$\"\"$ (Divide each side by 2)

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$\"\"$ (Cancel common terms)

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

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The value of $\"\"$ .