Question

Докажите равенство:

(а + b + с)2 = а2 + 2ab + b2 + 2аc + 2bс + с2

Answer 1

$(a + b + c)^2=[(a + b) + c]^2=(a+b)^2+2(a+b)c+c^2=a^2+2ab+2ac+2bc+c^2$