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The expression is $\"\"$ .

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$\"\"$ .

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Find the value of $\"\"$ .

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$\"\"$

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Sum formula of sine function : $\"\"$ .

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$\"\"$

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Thus, $\"\"$ .

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If $\"\"$ , then $\"\"$ .

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Therefore, $\"\"$ .

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