(abc)/(ab+bc+ac)=1/(ab+bc+ac)/(abc)=1/(1/a+1/b+1/c)
1/a+1/b=1/6,1/b+1/c=1/9,1/a+1/c=1/15所以1/a+1/b+1/c=(1/6+1/9+1/15)/2
abc)/(ab+bc+ac)=1/(ab+bc+ac)/(abc)=1/(1/a+1/b+1/c)=180/31