3. 令a=xy,b=x-y, f(a,b)=f(xy,x-y)=x^2+y^2=2a+b^2因此，此偏微分式为 2+2y4. dz=f'(u,v)df=df/du*(2dx+3dy)+df/dv(e^xydx+e^xydy)5.
