设函数f(x，y)=x/y，则df(x，y)=____．

设函数f(x，y)=x/y，则df(x，y)=____．
【正确答案】：(1/y)dx-(x/y²)dy. 解析:∵f(x,y)=x/y ∴∂f/ ∂x=1/y ∂f/ ∂y=-(x/y²) df(x，y)=(1/y)dx-(x/y²)dy．