$\"\"$

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The trigonometric identity $\"\"$ .

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Consider the L.H.S: $\"\"$ .

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Sum-to-product formula: $\"\"$ .

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$\"\"$

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$\"\"$

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$\"\"$

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Odd and even identities: $\"italic$ .

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$\"\"$

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Rewrite the expression.

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$\"\"$

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Sum-to-product formula: $\"\"$ .

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$\"\"$

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$\"\"$

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Therefore, $\"\"$ .

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$\"\"$

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$\"\"$ .