z=y/(y^2+x^2)^(1/2）z'=-(y/2)2x/(x^2+y^2)^(3/2) = -xy/(x^2+y^2)^(3/2).z'=(x^2+y^2)^(1/2)-y^2/(x^2+y^2)^(1/2)/(x^2+y^2)=x^2/(x^2+y^2)^(3/2).dz=x(-ydx+xdy)/(x^2+y^2)^(3/2)