ab/a+b=3
ca/c+a=4
bc/b+c=5
1/a+1/b=1/3
1/c+1/a=1/4
1/b+1/c=1/5
相加,
2(1/a+1/b+1/c)=47/60
1/a+1/b+1/c=47/120
ab+bc+ac/abc
=1/c+1/a+1/b
=47/120