ab=2(a+b),bc=3(b+c),ca=4(c+a),
(a+b)/(ab)=1/2,(b+c)/(bc)=1/3,(c+a)/(ca)=1/4
1/a+1/b=1/2
1/b+1/c=1/3
1/c+1/a=1/4
3式相加得:
1/a+1/b+1/c=13/24
1/a=5/24,a=24/5
1/b=7/24,b=24/7
1/c=1/24,c=24
（a-1）（b-1）（c-1）
=(19/5)(17/7)(23)
=7429/35