a)
(a+b+c)^2=a^2+2ab+b^2+2(a+b)*c+c^2=
=a^2+2ab+b^2+2(ac+bc)+c^2=
=a^2+2ab+b^2+2ac+2bc+c^2=
=a^2+2ab+2ac+b^2+2bc+c^2
b)
(a+b)^4=[(a+b)^2]^2=
=(a^2+2ab+b^2)^2=
=a^4+4a^3b+4a^2b^2+2(a^2+2ab)*b^2+b^4=
=a^4+4a^3b+4a^2b^2+2a^2b^2+4ab^3+b^4=
=a^4+4a^3b+6a^2b^2+4ab^3+b^4