In a+b, Ab = 1, so a+b=3ab, and when both sides are multiplied by C, ac+bc=3abc, the same is true.

B+c BC = 1，b+c=4bc，ab+ac=4abc。

A+c AC = 1，a+c=5ac，ab+bc=5abc。

Add the three formulas to get it.

(AC+BC)+(a b+AC)+(a b+BC)= 12 ABC

2(ab+bc+ac)= 12abc

ab+bc+ac=6abc

abc/(ab+bc+ac)= 1/6