(a-b+c+b) (a-b-c-d)=[(a-b)+(c+d)][(a-b)-(c+d)]=(a-b)^2-(c+d)^2=a^2-2ab+b^2-(c^2+2cd+d^2)=
=a^2-2ab+b^2-c^2-2cd-d^2
(a-b+c+b) (a-b-c-d)=[(a-b)+(c+d)][(a-b)-(c+d)]=(a-b)^2-(c+d)^2=a^2-2ab+b^2-(c^2+2cd+d^2)=
=a^2-2ab+b^2-c^2-2cd-d^2