f(x)=1/x²那么导数为f'(x)=lim (dx趋于0) [f(x+dx) -f(x)]/dx=lim (dx趋于0) [1/(x+dx)² -1/x²]/dx=lim (dx趋于0) [-(2xdx+dx²)/(x+dx)²x²] /dx=lim (dx趋于0) -(2x+dx)/(x+dx)²x²代入dx=0，得到f'(x)= -2/x^3
