x^2+y^2+z^2+xy+xz+zy=2(x^2+y^2+z^2+xy+xz+zy)/2=(2x^2+2y^2+2z^2+2xy+2xz+2zy)/2=(x^2+2xy+y^2+2zy+z^2+x^2+2xz+z^2)/2=[(x+y)^2+(y+z)^2+(x+z)^2]/2
(x+y+x)²/2
