\(\left(a+b+c\right)^2\)
\(=\left[\left(a+b\right)+c\right]^2\)
\(=\left(a+b\right)^2+2\left(a+b\right)c+c^2\)
\(=a^2+2ab+b^2+2ac+2bc+c^2\)
\(=a^2+b^2+c^2+2ab+2bc+2ac\)
\(\left(a+b+c\right)^2\)
\(=\left(a+b\right)^2+2\left(a+b\right)c+c^2\)
\(=a^2+2ab+b^2+2ac+2bc+c^2\)
\(=a^2+b^2+c^2+2\left(ab+bc+ac\right)\)
\(\left(a+b+c\right)^2=a^2+b^2+c^2+2ab+2bc+2ac\)
\(\left(a+b+c\right)^2\)
\(=\text{[}\left(a+b\right)+c\text{]}^2\)
\(=\left(a+b\right)^2+2\left(a+b\right)c+c\)
\(=a^2+2ab+b^2+2ac+2bc+c^2\)
\(=a^2+b^2+c^2+2ab+2bc+2ac\)