(a+b)^2=a^2+b^2+2ab
(a+b+c)^2=a^2+b^2+c^2+2ab+2ac+2bc
(a+b+c+d)^2=a^2+b^2+c^2+d^2+2ab+2ac+2az+2bc+2bz+2cz
(x+y+z+1)^2=x^2+y^2+z^2+1+2xy+2xz+2x+2yz+2y+2z