a^2+b^2+c^2-ab-bc-ca=1/2(2a^2+2b^2+2c^2-2ab-2ac-2bc)=1/2[(a-b)^2+(a-c)^2+(b-c)^2]因为a=1996x+1995,b=1996x+1996,c=1996x+1997a-b=-1a-c=-2b-c=-1所以=1/2[(a-b)^2+(a-c)^2+(b-c)^2]=1/2(1+4+1) ...