a的2次方+b的2次方+c的2次方—ab—bc—ac
=1/2[2a的2次方+2b的2次方+2c的2次方—2ab—2bc—2ac]
=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]