那个符号没法输入，用s表示当s足够小的时候，f(s)=f'(0)s +o(s)f'(0) = lim [(f(x)-f(0)]/x = lim 2f(s)/x =2lim f'(0)s /x =2f'(0)lim(s/x)lim s/x = 1/2
