$\"\"$

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The trigonometric expression is $\"\"$ .

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Rewrite the expression as $\"\"$ .

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

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$\"\"$

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Double-angle formula : $\"\"$ and $\"\"$ .

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$\"\"$

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Pythagorean identity : $\"\"$ .

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$\"\"$

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$\"\"$

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Therefore, $\"\"$ . $\"\"$ $\"\"$ .