SOLUTION: Rewrite the product as a sum or difference. 2 sin(9x) cos(2x)

You are probably more familiar with this more common formula than the 
general product-to-sum and sum-to-product formulas, so I prefer to use 
this more familiar one:







Add the two equations:



The left side is a sum.

Edwin

Use a general formula

    sin(a)*cos(b) = ,


which is valid for any angles "a" and "b".


In out case, a = 9x, b = 2x.  Therefore

    2*sin(9x)*cos(2x) = sin(9x-2x) + sin(9x+2x) = sin(7x) + sin(11x).    ANSWER