因为abc=1.,
所以a/(ab+a+1)+b/(bc+b+1）+c/（ca+c+1)
=a/(ab+a+abc)+b/(bc+b+1)+c/(ac+c+abc)
=1/(b+1+bc)+b/(bc+b+1)+1/(a+1+ab)
=1/(b+1+bc)+b/(bc+b+1)+abc/(a+abc+ab)
=1/(b+1+bc)+b/(bc+b+1)+bc/(1+bc+b)
=(1+b+bc)/(b+1+bc)
=1.