zx=1/y,代入y=1得zx=1zy=-（x/y^2)代入x=2，y=1得zy=-2所以dz=dx-2dy
Zx=1/yZy=-x/(y^2)dz(2,1)=1/1+-2/(1^1)=-1
dz=1/ydx-x/y²dy将（2,1）代入dz=dx-2dy
