1/a+1/b>=4/(a+b)
(b+a)(a+b)>=4ab
(a+b)^2>=4ab
a^2+2ab+b^2>=4ab
a^2-2ab+b^2>=0
(a-b)^2>=0
(1/2a)+(1/2b)+(1/2c)大于等于（1/(b+c))+(1/(c+a))+(1/(a+b)
通分,相减>=0