(a) Derive the product-to-sum formula for cos A cos B.
Chapter 8, Problem 42(choose chapter or problem)
(a) Derive the product-to-sum formula for cos A cos B. Hint: Add equations (1) and (2) in the text on page 585. (b) Derive the product-to-sum formula for sin A cos B. Hint: Start by writing down the two addition formulas for sin(A B) and sin(A B), then add the two equations.
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