1/a+1/b=1/6 1/b+1/c=1/9 1/a+1/c=1/15 求(abc)/(ab+bc+ca)的值

三个式子相加,有
2(1/a+1/b+1/c)=31/90
1/a+1/b+1/c=31/180
即
(ab+bc+ca)/(abc)=31/180
所以
(abc)/(ab+bc+ca)=180/31