代数abc=1a+b+c=2a^2+b^2+c^2=31/ab+
abc=1 a+b+c=2 a^2+b^2+c^2=3 1/ab+c-1+1/bc+a-1+1/ca+b-1=
原式=1/（ab+c-1）+1/（bc+a-1）+1/(ca+b-1) =1/(a-1)(b-1) +1/(b-1)(c-1) +1/(c-1)(a-1) =(a-1+b-1+c-1)/(a-1)(b-1)(c-1) =-1/3/2 =-2/3