`a) 18 vdots n`
`=> n in Ư(18)={1;2;3;6;9;18}`
`b) 2n +1 in Ư(18)={1;2;3;6;9;18}`
`=> 2n in {0;1;2;5;8;17}`
`=> n in {0;1;4}`
`c)` - Ta có : `2.3.5.7.11 vdots 3`
`21 vdots 3 => 13.17.21.19 vdots 3`
`=> 2.3.5.7.11+13.17.21.19 vdots 3`
mà `2.3.5.7.11+13.17.21.19 >3`
`=> 2.3.5.7.11+13.17.21.19` là hợp số