z'x=-xy/√(x^2+y^2)^3Z'y=1/√(x^2+y^2)-y^2/√(x^2+y^2)^3 =x^2/√(x^2+y^2)^3dz=z'x dx+z'ydy =(-xy)/√(x^2+y^2)^3 dx + (x^2)/√(x^2+y^2)^3 dy
