1. The problem statement, all variables and given/known data If x=yz and y=2sin(y+z), find dx/dy 2. Relevant equations Chain rule 3. The attempt at a solution From y = 2sin(y+z) we get dz/dy= (1-2cos(y+z))/(2cos(y+z)) dz/dy=((1/2)sec(y+z) - 1) dx/dy = ∂x/∂y + ∂x/∂z dz/dy = z + y ((1/2)sec(y+z) - 1) = z - y - (1/2) sec(y+z) But the answer is z-y+tan(y+z) (Mathematical methods in the physical sciences, Boas, Ch. 4 Sec 7 Problem 1) What is going wrong?