$\"\"$

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The trigonometric identity $\"\"$ .

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Consider left hand side of the identity $\"\"$ .

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Apply the sum-to-product formula: $\"\"$ .

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Apply the sum-to-product formula: $\"\"$ .

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$\"\"$

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$\"\"$

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Apply reduction formulas: $\"\"$ and $\"\"$ .

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$\"\"$

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$\"\"$

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Apply quotient identity: $\"\"$ .

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$\"\"$ .

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$\"\"$ Right hand side of the identity.

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$\"\"$ .

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$\"\"$

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$\"\"$ .