a+a chia hết 6+a

`a + a` $\vdots$ `6 + a`

`=> 2a` $\vdots$ `6 + a`

`=> 2(6 + a) - 2.6` $\vdots$ `6 + a`

`=> 2.6` $\vdots$ `6 + a`

`=> 12` $\vdots$ `6 + a`

`=> 6 + a in Ư(12)`

`=> Ư(12) in {+-1; +-2; +-3; +-4; +-6; +-12}`

`=> 6 + a in {+-1; +-2; +-3; +-4; +-6; +-12}`

`=> a in {-5;-7;-4;-8;-3;-9;-2;-10;0;-12;6; -18}`

Vậy `a in {-5;-7;-4;-8;-3;-9;-2;-10;0;-12;6; -18}` để `a + a` $\vdots$ `6 + a`