所求=(2a^2+2b^2+2c^2-2ab-2ac-2bc)/2
=(a^2+b^2-2ab+a^2+c^2-2ac+b^2+c^2-2bc)/2
=[(a-b)^2+(a-c)^2+(b-c)^2]/2
=[1+4+1]/2
=3