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Why is [math]\cos\alpha \cos\beta=\frac{1}{2}[\cos(\alpha-\beta)+\cos(\alpha+\beta)][/math]?
What are the product-to-sum identities?
The product-to-sum identities are trigonometric formulas that express the product of two trigonometric functions as a sum or difference of trigonometric functions. Some examples include [math]\cos x\cos y=\frac{1}{2}[\cos(x-y)+\cos(x+y)][/math] and [math]\sin x \cos y=\frac{1}{2}[\sin(x+y)+\sin(x-y)][/math].