z=arctan[x/(1+y²)]
∂z/∂x={1/[1+x²/(1+y²)]}*[1/(1+y²)]
∂z/∂y={1/[1+x²/(1+y²)]}*[-x/(1+y²)²]*2y
所以dz={1/[1+x²/(1+y²)]}*[1/(1+y²)]dx+{1/[1+x²/(1+y²)]}*[-x/(1+y²)²]*2ydy
x=1,y=1
dz=(2/5)dx-(2/5)dy