a. $\overline{abcabc}$ + `22`
Vì $\overline{abcabc}$ $\vdots$ `11`
`22` $\vdots$ `11`
-> $\overline{abcabc}$ + `22` $\vdots$ `11`
=> $\overline{abcabc}$ + `22` là hợp số
b. $\overline{abcabc}$ + `39`
$\overline{abcabc}$ = $\overline{abc}$ . `1001` = $\overline{abc}$ . `7 . 11 . 13`
`39` $\vdots$ `13`
-> $\overline{abcabc}$ + `39` $\vdots$ `13`
-> $\overline{abcabc}$ + `39` là hợp số
