(根号a-根号b)的平方≥0
乘开来就是a+b-2根号(ab)≥0
即1=a+b≥2根号(ab),ab≤1/4
(1+1/a)(1+1/b) = (1+a)(1+b)/ab = 2/ab+1 ≥ 2/(1/4)+1 = 9