a=2002x+2001,b=2002x+2002,c=2002x+2003 ∴a-b=-1,b-c=-1,c-a=2 ∴a2+b2+c2-ab-ac-bc =1/2[2a2+2b2+2c2-2ab-2ac-2bc] =1/2[（a-b)^2+(b-c)^2+(c-a)^2] =1/2[（-1)^2+(-1)^2+2^2] =3