Answer
$a^{2}+b^{2}+c^{2}+2ab+2bc+2ac$
Work Step by Step
Addition is associative, we can write $a+b+c$
as a sum of two terms:
$(a+b+c)^{2}=[(a+b)+c]^{2}$
... and we now square the binomial
$=(a+b)^{2}+2(a+b)(c)+c^{2}$
... square the binomial, distribute $2c(a+b)$...
$=a^{2}+2ab+b^{2}+2ac+2bc+c^{2}$
... simplify
=$a^{2}+b^{2}+c^{2}+2ab+2bc+2ac$
