Ta có : \(4^2 = (-4)^2 = 16\)
$\begin{array}{l}
{\left( {{x^2} + 3} \right)^2} = 16\\
\Rightarrow \left[ \begin{array}{l}
{x^2} + 3 = 4\\
{x^2} + 3 = - 4
\end{array} \right. \Rightarrow \left[ \begin{array}{l}
{x^2} = 4 - 3\\
{x^2} = - 4 - 3
\end{array} \right.\\
\Rightarrow \left[ \begin{array}{l}
{x^2} = 1\\
{x^2} = - 7\,\,(vo\,\,ly)
\end{array} \right.\\
\Rightarrow {x^2} = 1\\
\Rightarrow \left[ \begin{array}{l}
x = 1\\
x = - 1
\end{array} \right.
\end{array}$
