(a+b)^2=a^2+b^2+2ab=3ab+2ab=5ab
(a-b)^2=a^2+b^2-2ab=3ab-2ab=ab
[(a+b)/(a-b)]^2=5ab/(ab)=5
因为 a＞b＞0
所以(a+b)/(a-b)= 根号5