由已知得:e^(x+y)=xy.
d e^(x+y)=dxy.
e^(x+y)*d(x+y)=(ydx+xdy).
e^(x+y)*(dx+dy)=ydx+xdy.
e^(x+y)dx+e^(x+y) dy=ydx+xdy.
[(e^(x+y)]dy-xdy=[y-e^(x+y)]dx.
dy={[y-e^(x+y)]/[e^(x+y)-x]}dx.
令F(x,y)=e^(x+y)-xy
有隐函数求导
dy/dx=(dF/dx) / (dF/dy)
=(e^(x+y)-y) / (e^(x+y)-x)
于是dy={ [y-e^(x+y)]/[e^(x+y)-x]} dx.