ac<=(a+c)(a+c)/4

简单啦!abc/(ab+bc+ac)=2abc/2(ab+bc+ac)=2abc/ab+bc+ac+ab+bc+ac=2abc/a(c+b)+b(a+c)+c(a+b)
ab/(a+b)=1/3 a+b=3ab
bc/(b+c)=1/4 b+c=4bc
ac/(a+c)=1/5 a+c=5ac