Bài 5 :
a) \(3\) chia hết cho \(n+5\)
\( \Rightarrow n +5 \in U(3)\)
\( \Rightarrow n+5 \in \{ - 3 ; -1 ; 1 ; 3\}\)
\(+) \; n+5 =-3 \Rightarrow a = -3- 5 =-8 \;\)
\(+) \; n+5 = -1\Rightarrow a = -1-5 = -6\;\)
\(+) \; n+5 = 1\Rightarrow a =1 -5 =-4\;\)
\(+) \; n+5 = 3\Rightarrow a =3 -5 =-2 .\)
Vậy \(n \in \{-8; -6; -4; -2\}\).