点(x, y)沿平面直线y=x趋于(0, 0)的情形lim(x→0, y=x) [xy/(x+y)]=lim(x→0) (x²/2x)=0点(x, y)沿平面直线y=-x趋于(0, 0)的情形lim(x→0, y=-x) [xy/(x+y)]=lim(x→0) [-x²/(x-x)]→∞∴命题得证...