a,
`a^2(a+1)+2a(a+1)`
`=(a+1)(a^2+2a)`
`=a(a+1)(a+2)`
Vì `a(a+1)(a+2)` là tích `3` số nguyên liên tiếp
`=>`$\begin{cases}a(a+1)(a+2)\vdots2\\a(a+1)(a+2)\vdots3\\\end{cases}$
`=>a(a+1)(a+2)\vdots6`
b,
`a(2a-3)-2a(a+1)`
`=a(2a-3)-a(2a+2)`
`=a(2a-3-2a-2)`
`=-5a\vdots5`