lim趋于4(3-√(5+x)/1-√(5-x))第一步怎么化简

x→4lim[(3-√(5+x)]/[1-√(5-x)]
=x→4lim[3-√(5+x)][(3+√(5+x)][1+√(5-x)]/[1-√(5-x)][1+√(5-x)][(3+√(5+x)]
=x→4lim[9-(5+x)][1+√(5-x)]/[1-(5-x)][3+√(5+x)]
=x→4lim(4-x)[1+√(5-x)]/(-4+x)[3+√(5+x)]
=x→4lim{-[1+√(5-x)]/[3+√(5+x)]}
=(-2)/6=-1/3

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）