`a)` gọi 3 STN đó là: `a + 1 + a + 2 + a + 3 = a . 3 + (1 + 2 + 3) = a . 3 + 6`
Mà `3 ; 6` $\vdots$ `3
`=> a + 1 + a + 2 + a + 3` $\vdots$ `3`
`b)` `aaaaaa = a . 111111`
Mà `111111` $\vdots$ `7`
`=> aaaaaa` $\vdots$ `7`
c)`abcabc = abc . 1001`
Mà `1001` $\vdots$ `11`
`=> abcabc` $\vdots$ `11`