`8)`
`1+5+5^2+..............+5^10+5^11+5^12`
`=(1+5)+(5^2+5^3)+.................+(5^9+5^10)+(5^11+5^12)`
`=6.1+ 6.5^2+...................+6.5^9+6.5^11`
`=6.(1+5^2+...............+5^9+5^11)`
Vì `31:6=5(` dư `1)`
`=>6.(1+5^2+...............+5^9+5^11):31=5(` dư `1)`
`=>(1+5+5^2+..............+5^10+5^11+5^12):31=5(` dư `1)`
`=>` Chọn `1`
`color{blue}\text{sát thủ 2.0}`