limf''(x)/|x|=1表明在x=0附近（即某邻域）f''(x)/|x|>0, 从而f''(x)>0, 从而f'(x)递增, 从而当x<0时, f'(x)0时, f'(x)>f'(0)=0, 所以f(0)是极小值
