`Ta` `có`: $5n -3$ chia `hết` `cho` $2n+1$
`=>` $2(5n-3)$ chia `hết` `cho` $2n+1$
`=>` $10n - 6$ chia `hết` `cho` $2n+1$
`=>` $5(2n+1)-11$ chia `hết` `cho` $2n+1$
`Mà` $2n+1$ chia `hết` `cho` $2n+1$
`=>` $5(2n+1)$ chia `hết` `cho` $2n+1$
`=>` $11$ chia `hết` `cho` $2n+1$
`=>` $2n+1$ ∈ Ư(11) = { ±1; ±11 }
`=>` $2n$ ∈ { 0; -2; 10; -12 }
`=>` $n$ ∈ { 0; -1; 5; -6 }
`Vậy` $n$ ∈ { 0; -1; 5; -6 }
