a^2+b^2+c^2-ab-bc-ac =1/2[(a^2-2ab+b^2)+(a^2-2ac+c^2)+(b^2-2bc+c^2)] =1/2[(a-b)^2+(a-c)^2+(b-c)^2] =1/2[1^2+(-1)^2+(-2)^2] =3 答案选B
