$\"\"$

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The identity is $\"\"$ .

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Consider left hand side identity : $\"\"$ .

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Quotient identity : $\"\"$ .

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$\"\"$

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$\"\"$

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Apply sine sum identity : $\"\"$ .

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$\"\"$

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Apply cosine sum identity : $\"\"$ .

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$\"\"$ (Since $\"\"$ )

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Apply sine difference identity : $\"\"$ .

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Apply cosine difference identity : $\"\"$ . \ \

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$\"\"$

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= Right hand side identity.

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$\"\"$

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$\"\"$ .