\[\left( (a + b) + c \right)^{2} =\]
\[= (a + b)^{2} + 2 \cdot (a + b)c + c² =\]
\[= a^{2} + 2ab + b^{2} + 2ac + 2bc + c^{2} =\]
\[= a^{2} + b^{2} + c^{2} + 2ab + 2ac + 2bc.\]