1/a+1/b=(a+b)/ab=3;1/b+1/c=(b+c)/bc=4;1/c+1/a=(c+a)/ca=5
2(1/a+1/b+1/c)=3+4+5=12
1/a+1/b+1/c=6
abc/(ab+bc+ca)=1/(1/a+1/b+1/c)=1/6
1/a+1/b=(a+b)/ab=3;1/b+1/c=(b+c)/bc=4;1/c+1/a=(c+a)/ca=5
2(1/a+1/b+1/c)=3+4+5=12
1/a+1/b+1/c=6
abc/(ab+bc+ca)=1/(1/a+1/b+1/c)=1/6