According to the definition of limit, lim(x→+∞)f'(x)=A means that for any E>0, x > exists; 0, when x>X | f' (x)-a |

Take E=A/2, then there is x, when x >；; X has | f' (x)-a |

The local number is saved based on A/2 >; 0，f '(x)& gt； 0, but it is actually f' (x) >; A/2。