Vì: `a` chia 3 dư 1 suy ra: `a=3k+1(k\inZ)`

`b` chia 3 dư 2 suy ra: `b=3q+2(q\inZ)`

`=>ab=(3k+1)(3q+2)`

`=3k(3q+2)+(3q+2)*1`

`=9kq+6k+3q+2`

`=3(3kq+2k+q)+2`

Mà: `3(3kq+2k+q)\vdots3`

`=>3(3kq+3k+q)+2` chia `3` dư `2`

Hay: `ab` chia 3 dư 2 

Vậy: ...