f(x)在x=0处连续，且lim(x→0)f(x)/x存在,假设价值为A∵分母x→0∴分子f(x)→0(否则极限不存在)∴lim(x→0)f(x)/x为0/0型lim(x→0)f(x)/x=lim(x→0)f'(x)/x'=f'(0)=A∴f(x)在x=0处可导
