$\\$
`a,`
`n+4 \vdots n+1`
`-> n+1+3 \vdots n+1`
Vì `n+1 \vdots n+1`
`-> 3 \vdots n+1`
`-> n+1 ∈ Ư (3) = {1;-1;3;-3}`
`-> n ∈ {0; -2; 2;-4}`
Vì `n ∈ NN`
`-> n ∈ {0; 2}`
Vậy `n ∈ {0;2}` để `n+4 \vdots n+1`
$\\$
`b,`
`n^2 + 4 \vdots n+2`
`-> n^2 -2n + 2n - 4 + 8 \vdots n +2`
`-> (n^2- 2n) + (2n-4) + 8 \vdots n+2`
`-> n (n-2) + 2 (n-2) + 8 \vdots n+2`
`-> (n-2) (n+2) + 8 \vdots n+2`
Vì `n+2 \vdots n+2 -> (n-2) (n+2) \vdots n+2`
`-> 8 \vdots n+2`
`-> n+2 ∈ Ư ( 8)={1;-1;2;-2;4;-4;8;-8}`
`-> n ∈ {-1; -3; 0; -4; 2;-6; 6;-10}`
Vì `n ∈ NN`
`-> n ∈ {0;2;6}`
Vậy `n ∈ {0;2;6}` để `n^2 + 4 \vdots n+2`
$\\$
`c,`
`13n \vdots n-1`
`-> 13n - 13 + 13 \vdots n-1`
`-> 13 (n-1) + 13 \vdots n-1`
Vì `n-1 \vdots n-1 -> 13 (n-1) \vdots n-13`
`-> 13 \vdots n-1`
`->n-1 ∈ Ư (13) = {1;-1;13;-13}`
`-> n ∈ {2; 0; 14; -12}`
Vì `n ∈ NN`
`->n ∈ {2;0;14}`
Vậy `n ∈ {2;0;14}` để `13n \vdots n-1`
