`2n^² + 5n - 1vdots2n-1`
`ton^²-n +6n-3+2vdots2n-1`
`ton(2n-1) +3(2n-1)+2vdots2n-1`
`to(2n-1) (n+3)+2vdots2n-1`
`to2vdots2n-1`
`to2n-1\in Ư_((2))={+-1;+-2}`
`to2n\in{-1;0;2;3}`
`tonin{(-1)/2;0;1;3/2}`
`text(và )ninmathbbZ`
`tonin{0;1}text( thì )2n^2+5-1vdots2n-1`