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