(a+b+c+d)^2
=[(a+b)+(c+d)]^2
=(a+b)^2+(c+d)^2+2(a+b)(c+d)
=a^2+b^2+2ab+c^2+d^2+2cd+2ac+2ad+2bc+2bd
=a^2+b^2+c^2+d^2+2ab+2cd+2ac+2ad+2bc+2bd