$\"\"$

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The trigonometric identity is $\"\"$ .

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Left hand side identity : $\"\"$ .

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Since the cosine is an even function, $\"\"$ .

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$\"\"$

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Apply quotient identity : $\"\"$ .

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$\"\"$

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$\"\"$

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= Right hand side identity.

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$\"\"$

\

Thus, $\"\"$ .