The expression 'cos a sin b' refers to the product of the cosine of angle 'a' and the sine of angle 'b'. This expression is significant in trigonometry, especially in the context of product-to-sum and sum-to-product identities, as it provides a way to convert the product of these two trigonometric functions into a sum or difference form, which can simplify calculations and help solve various trigonometric equations.
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