a^2+b^2+c^2-bc-ba-ca
=1/2(2a^2+2b^2+2c^2-2bc-2ba-2ca)
=1/2[(a^2-2ab+b^2)+(b^2-2bc+c^2)+(c^2-2ca+a^2)]
=1/2[(a-b)^2+(b-c)^2+(c-a)^2]