`A=4^0+4^1+4^2+.....+4^9+4^10`
`A=1+4^1+4^2+4^3+........+4^9+4^10`
`A=(1+4)+(1+4).4^2+..........+(1+4).4^9`
`A=5+5.4^2+...........+5.4^9`
`A=5.(1+4^2+..........+4^9)`
⇒A chia hết cho 5
Bài 2
Do 10 chia hết cho n-2
`⇒n-2 ∈ ƯC(10)`
mà `ƯC(10) ∈ {-10,-8,-6,-4,-2,0,2,4,6,8,10}`
`⇒n-2 ∈ {-10,-8,-6,-4,-2,0,2,4,6,8,10}`
$\left[\begin{array}{ccc}-10&-8&-6&-4&-2&0&2&4&6&8&10\\-8&-6&-4&-2&0&2&4&6&8&10&12\end{array}\right]$
mà n là stn `⇒ n ∈ {0,2,4,6,8,10,12}`
Vậy `n ∈ {0,2,4,6,8,10,12}`