$\"\"$

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The identity is $\"\"$ .

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

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Rewrite the identity.

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$\"\"$

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Apply Cosine double angle identity : $\"\"$ .

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$\"\"$

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$\"\"$

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Apply Sine double angle identity : $\"\"$ .

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$\"\"$

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$\"\"$

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$\"\"$ .

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$\"\"$

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$\"\"$ .