$\"\"$

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The expression is $\"\"$ .

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$\"\"$

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$\"\"$ (Apply half-angle formula)

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

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

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$\"\"$

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$\"\"$

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$\"\"$ $\"\"$

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