In the problem 4cosxsinx...how would you deal with the 4 in the equation? I understand how to work out and multiply with cosx sinx but what about the constant? Solution 4 is constant just let it be in multiplication and applying product rule and see.... since the derivative of a constant is 0. d(4cosxsinx)/dx= d(4)/dx sinx cosx + d(cosx)/dx 4sinx + d(sinx)/dx 4cosx = 0 -4(sinx)^2 + 4 (cosx)^2 = 4[(cos x)^2-(sin x)^2]= 4 cos2x.