$\"\"$

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Sum-to-Product Formula (9) is $\"\"$ .

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

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Apply the Product-to-Sum formula: $\"\"$ .

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$\"\"$

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$\"\"$

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= Left hand side of the identity.

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Thus, $\"\"$ .

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Sum-to-Product formula (9) is derived.

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$\"\"$

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$\"\"$ .