左右极限不相同,左极限为-1 右极限为1
lim(x-1)Ix-1I/(x-1) 当x-→1+(x从1的右边趋近) =lim(x-1)/(x-1)=1
当x-→1-(x从1的左边趋近) =lim(1-x)/(x-1)=-1