a^2+b^2+c^2-ab-bc-ac
=1/2(a^2-2ab+b^2)+1/2(b^2-2bc+c^2)+1/2(a^2-2ac+c^2)
=1/2(a-b)^2+1/2(b-c)^2+1/2(a-c)^2
就这些了,你的原题没有这个等于0之类的吗?