X^2+Y^2+Z^2-XY-XZ-YZ =1/2*(2X^2+2Y^2+2Z^2-2XY-2XZ-2YZ) =1/2*(x^2+y^2-2xy+y^2+z^2-2yz+x^2+z^2-2xz) =1/2((x-y)^2+(y-z)^2+(x-z)^2)