$\"\"$

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The expression is $\"\"$ .

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Let $\"\"$ , then $\"\"$ .

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$\"\"$ .

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Use Pythagorean identity to find the value of $\"\"$ .

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Pythagorean identity : $\"\"$ .

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$\"\"$

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Quotient identity : $\"\"$ .

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$\"\"$

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$\"\"$

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The exact value of $\"\"$ is $\"\"$ .