abc=1ab=1/ca=1/bcac=1/b所1/(1+a+ab)+1/(1+b+bc)+1/(1+c+ac)=1/(1+1/bc+1/c)+1/(1+b+bc)+1/(1+c+1/b)第乘bc第三乘b=bc/(1+b+bc)+1/(1+b+bc)+b/(1+b+bc)=(1+b+bc)/(1+b+bc)=
