δf/δx=1/(1+(x/y)²)* 1/y=y/(x²+y²)δf/δy=1/(1+(x/y)²)* (-x/y²)=-x/(x²+y²)y'=x/√x^2+1所以dz/dx=δf/δx+δf/δy*y'=y/(x²+y²)-x/(x²+y²)* x/(√x^2+1)=1/(x²+y²)*【y-x²/(√x²+1)】
