a: \(\Leftrightarrow\left(n+3\right)\left(n-3\right)⋮n^2+3\)
\(\Leftrightarrow n^2+3-12⋮n^2+3\)
\(\Leftrightarrow n^2+3\in\left\{3;4;6;12\right\}\)
hay \(n\in\left\{0;1;3\right\}\)
c: \(\Leftrightarrow n^2-4⋮n^2-1\)
\(\Leftrightarrow n^2-1\in\left\{1;-1;3;-3\right\}\)
hay \(n\in\left\{0;2\right\}\)