f(x+y,xy)=x²+y²+xy
=x²+y²+2xy-xy
=(x+y)²-xy
f(x,y)=x²-y
df(x,y)/dx=2x
df(x,y)=2xdx
df(x,y)/dy=-1
df(x,y)=-dy
所以
df(x,y)==2xdx-dy