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The product of two trigonometric functions of two angles that are expressed in terms of the sum of the trigonometric functions of the sum of the two angles. They are useful when simplifying trigonometric expressions.
sinx cosy =(1/2)[sin(x + y) + sin(x - y)] cosx siny = (1/2)[sin(x + y) - sin(x - y)] cosx cosy = (1/2)[cos(x + y) + cos(x - y)] sinx siny = (1/2)[cos(x - y) - cos(x + y)]
