$\"\"$

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The expression is $\"\"$ .

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Rewrite the expression.

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$\"\"$

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Apply sum rule of cosine function : $\"\"$

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$\"\"$

\

$\"\"$

\

$\"\"$

\

$\"\"$

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Therefore,the exact value of the expression is $\"\"$ .

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$\"\"$

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