1. The problem statement, all variables and given/known data find dz/dy(partial) z= tan-1(y/x) 2. Relevant equations 3. The attempt at a solution z= tan-1(y/x) let u=y/x z= tan-1(u) dz/du = 1/ (1+u2) so dz/dy = dz/du (du/dy) du/dy = 1/x so dz/dy = (1/1+u2)(1/x) = 1/ 1+ (y2/x2) * 1/x = 1/ x + (y2/x2)x) = x / x + y2 but I should be getting... dz/dy = x / x2 + y2 but I found dz/dx the same way and it was the right answer.? whoops I made a arithmetic mistake it is correct. sorry.