答：ab/(a+b)=1,a+b=ab
bc/(b+c)=1/2,b+c=2bc
ac/(a+c)=1/3,a+c=3ac
三式相加：
2(a+b+c)=ab+2bc+3ac
=a(b+c)+2(a+b)c
=2abc+2abc
=4abc
所以：
a+b+c=2abc
所以：abc/(a+b+c)=1/2=ab+2bc+3ac=a(b+c)+2(a+b)c？？咋么变得？？把3ac拆开成2ac+ac=a(b+c)+2(a+b)c=2abc+2abc呢么这里呢？？把最上面得出来的：b+c=2bca+b=ab代入进去啊