lim(x→4)〖(3-√(5+x))/(1-√(5-x))〗 分子分母均同乘以〖(3+√(5+x))(1+√(5-x))〗 得到
=lim(x→4)〖(3-√(5+x))/(1-√(5-x))〗
= lim(x→4)〖-(1+√(5-x))/(3+√(5+x))〗
= -2/6
=-1/3
（其中约去了x-4和4-x这个0因子,所以多出个负号）