`3n+1` $\vdots$ `11-2n`
`=> 6n+2` $\vdots$ `11-2n`
và `3(11-2n)` $\vdots$ `11-2n`
`=> 6n+2+33-6n `$\vdots$ `11-2n`
`=> 35` $\vdots$ `11-2n`
do `n∈N=> 2n>=0<=>11-2n<=11`
`=> 11-2n∈{-35;-7;-5;-1;1;5;7}`
`<=> 2n∈{46;18;16;12;10;6;4}`
`<=> n∈{23;9;8;6;5;3;2}`