cos(8x)-8x*sin(8x). We use the product rule and power rule in this problem. The differential of a function y=f*g is f'*g+f*g', where the ’ symbol of a function means its differential. In our case, the differential of x is just 1, and the differential of cos(8x) is -8sin(8x) according to the power rule. Therefore, the differential of y is 1*cos(8x)+x*(-8sin(8x)).