a/ab+a+1)+（b/bc +b+1）+（c/ac+c+1）
=a/(ab+a+1) +ab/a(bc+b+1)+abc/ab(ac+c+1) 这步最关键
=a/(ab+a+1) +ab/(abc+ab+a)+1/(abc×a+abc+ab) abc=1
=a/(ab+a+1)+ab/(1+ab+a)+1/(a+1+ab)
=(a+ab+1)/(ab+a+1)
=1