Đáp án+Giải thích các bước giải:
a)$(a+b+c)^{2}$
=$[(a+b)+c]^{2}$
=$(a+b)^{2}$+2.(a+b).c+ $c^{2}$
=$a^{2}$+2ab+ $b^{2}$+2ac+2bc+ $c^{2}$
= $a^{2}$+ $b^{2}$+ $c^{2}$+2ab+2bc+2ac
b)$(a+b-c)^{2}$
=$[(a+b)-c]^{2}$
=$(a+b)^{2}$-2.(a+b).c+ $c^{2}$
=$a^{2}$+2ab+ $b^{2}$-2ac-2bc+ $c^{2}$
= $a^{2}$+ $b^{2}$+ $c^{2}$+2ab-2bc-2ac
c)$(a-b-c)^{2}$
=$[(a-b)-c]^{2}$
=$(a-b)^{2}$-2.(a-b).c+ $c^{2}$
=$a^{2}$-2ab+ $b^{2}$-2ac+2bc+ $c^{2}$
= $a^{2}$+ $b^{2}$+ $c^{2}$-2ab+2bc-2ac
