令x+y=m，x-y=n，那么x²-y²=(x+y)(x-y)=mn∴f(m,n)=mn，即f(x,y)=xy∴δf(x,y)/δx=y，δf(x,y)/δy=x∴δf(x,y)/δx+δf(x,y)/δy=x+y
