因为(a-1/2)^2>=0,故a^2>=a-1/4,于是a^2+b+3/4>=a+b+1/2=(a+1/4)+(b+1/4)>=2根号((a+1/4)(b+1/4))=根号[(2a+1/2)(2b+1/2)],同理可得其它两个类似不等式,三者相乘得结论.