lim [f(x0+h)-f(x0-h)]/2h
凑成导数定义就是
= lim [f((x0-h)+2h)-f((x0-h)-0)]/2h
把2h看成delta-x
= lim [f'(x0-h)]
h-->0
=f'(x0) 看懂了，谢谢