因为abc=1
所以1/(ab+a+1)=c/(abc+ac+c)=c/(ac+c+1)
1/(bc+b+1)=ac/(abc^2+abc+ac)=ac/(ac+c+1)
因此原式=c/(ac+c+1)+ac/(ac+c+1)+1/(ac+c+1)=(ac+c+1)/(ac+c+1)=1