`A =` $\dfrac{n+1}{n+3}$ `= 1 -`$\dfrac{2}{n+3}$

Để `A` $\in$ $\mathbb{Z}$ thì $\dfrac{2}{n+3}$

 `=> n +3` $\in$ {$\pm$ `1,` $\pm$ `2`}

Suy ra: `n = -2, -4, -1, -5`

Vậy: `n` $\in$ `{-5, -4,-2, -1}`