lim[f(x＋h)－f(x－h)]/(2h)→f'(x),则：lim[f(x＋h)－f(x－h)]/h→2f'(x)=－4为什么lim[f(x＋h)－f(x－h)]/(2h)→f'(x)lim[f(x＋h)－f(x－h)]/[(x＋h)－(x－h)]→f'(x)