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