\[\left( a^{2} + b^{2} \right)^{2} =\]
\[= \left( a^{2} \right)^{2} + 2a^{2}b^{2} + \left( b^{2} \right)^{2} =\]
\[= a^{4} + 2a^{2}b^{2} + b^{4}\text{.\ }\]