1. The problem statement, all variables and given/known data Differentiate twice: z = sinx 2. Relevant equations Product rule Chain rule 3. The attempt at a solution dy/dx = dy/dz * dz/dx dy/dx = dy/dz * cosx Using the product rule: d^2y/dx^2 = d^2y/dz^2 * cosx - dy/dz * sinx According to the answer in the book the answer is: d^2y/dx^2 = d^2y/dz^2 * cos^2x - dy/dz * sinx but I don't see how. Thanks for any help.