因为 AB/A+B=1/3 AC/A+C=1/4 BC/B+C=1/5
所以 A+B/AB=3 A+C/AC=4 B+C/BC=5
A+B/AB + A+C/AC + B+C/BC =3+4+5=12
2(AB+AC+BC)/ABC =12
AB+BC+AC/ABC =6
ABC/AB+AC+BC =1/6