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