abc=1,则
a/(ab+a+1)+b/(bc+b+1)+c/(ca+c+1)
=a/(ab+a+abc)+b/(bc+b+1)+bc/(cba+bc+b)
=1/(b+1+bc)+b/(bc+b+1)+bc/(1+bc+b)
=(1+b+bc)/(1+b+bc)
=1