$a$) $n+4 \vdots n$
$⇔ n + 4 - n \vdots n$
$⇔ 4 \vdots n$
$⇒ n$ $∈$ `Ư(4)={±1;±2;±4}`
Vậy $n$ $∈$ `Ư(4)={±1;±2;±4}`
$b$) $3n+7 \vdots n$
$⇔ 3n+7 - 3n \vdots n$
$⇔ 7 \vdots n$
$⇒$ $n$ $∈$ `Ư(7)={±1;±7}`
Vậy $n$ $∈$ `Ư(7)={±1;±7}`
$c$) `27 - 5n \vdots n`
`⇔ 27 - 5n + 5n \vdots n`
`⇔ 27 \vdots n`
`⇒` `n` `∈` `Ư(27)={±1;±3;±9;±27}`
Vậy `n` `∈` `Ư(27)={±1;±3;±9;±27}`