`#Sad`
`A= n^4-2n^3-n^2+2n`
`= n^4-n^3+3n^3+2n^2-3n^2-2n`
`= (n^4-n^3)+(3n^3-3n^2)+(2n^2-2n)`
`= n^3(n-1)+3n^2(n-1)+2n(n-1)`
`= (n^3+3n^2+2n)(n-1)`
`= (n^3+2n^2+n^2+2n)(n-1)`
`= [(n^3+2n^2)+(n^2+2n)](n-1)`
`= [n^2(n+2)+n(n+2)](n-1)`
`= (n^2+n)(n+2)(n-1)`
`= n(n+1)(n+2)(n-1)`
`= (n-1)n(n+1)(n+2)` `→` `(n in ZZ)`
`\text{→Bốn số nguyên liên tiếp sẽ}` $\vdots$ `24`
`⇒ (n-1)n(n+1)(n+2)` $\vdots$ `24`
`⇔ n^4-2n^3-n^2+2n` $\vdots$ `24`
