1. The problem statement, all variables and given/known data Find f'(x) for f(x) = sin x + 2 cos^3 x 2. Relevant equations Other than basic derivative properties and formulas, no. 3. The attempt at a solution f'(x) = sin x + 2cos^3 x = cos x - 6sin^2 x The book says the answer is f'(x) = cos x - 6cos^2 x sin x however I don't understand where the sin x comes from... any help?