因为 a/√b+ √b≥2√[(a/√b)•√b]=2√a
b/√a+ √a≥2√[(b/√a)•√a]=2√b
相加,得 a/√b+ b/√a+√a+ √b≥2√a+2√b
即 a/√b+ b/√a≥√a+√b