Answer
`(2n + 1)^2 = 5^2`
`=>` $\left[\begin{matrix} 2n + 1 = 5\\ 2n + 1 = -5\end{matrix}\right.$
`=>` $\left[\begin{matrix} 2n = 5 - 1\\ 2n = -5 - 1\end{matrix}\right.$
`=>` $\left[\begin{matrix} 2n = 4\\ 2n = -6\end{matrix}\right.$
`=>` $\left[\begin{matrix} n = 4 : 2\\ n = -6 : 2\end{matrix}\right.$
`=>` $\left[\begin{matrix} n = 2\\ n = -3\end{matrix}\right.$
$\text{Vậy}$ `n \in {2 ; -3}`
