SOLUTION: Express (sin 3x sin x) as a sum.

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Question 1171121: Express (sin 3x sin x) as a sum.
Answer by ikleyn(52858) About Me  (Show Source):
You can put this solution on YOUR website!
.

Use one of the basic Trigonometry formulas for the product of sine functions


    sin%28a%29%2Asin%28b%29 = %281%2F2%29%2A%28cos%28a-b%29+-+cos%28a%2Bb%29%29.


In your case  a = 3x;  b = x;  therefore


    sin(3x)*sin(x) = %281%2F2%29%2A%28cos%283x-x%29+-+cos%283x%2Bx%29%29 = %281%2F2%29%2Acos%282x%29 - %281%2F2%29%2Acos%284x%29,


exactly as requested.

Solved.