a>0.b>0,c>0 a+b+c=1,请证明1/(a+b)+1/(b+c)+1/(a+c)≥9/2

解，法一：因为2(a+b+c)=2,所以由柯西不等式
[(a+b)+(b+c)+(c+a)][1/(a+b)+1/(b+c)+1/(a+c)]>=
(1+1+1))^2=9
即2[1/(a+b)+1/(b+c)+1/(a+c)]>=9
所以1/(a+b)+1/(b+c)+1/(a+c)>=9&