Tham khảo
` S=3+3^2+3^3+...+3^{2021}`
`⇒S=(3+3^2)+(3^3+3^4+3^5)+....+(3^{2019}+3^{2020}+3^{2021})`
`⇒S=12+3^3.(1+3+3^2)+...+3^{2019}.(1+3+3^2)`
`⇒S=12+(1+3+3^2).(3^3+...+3^{2019})`
`⇒S=12+13.(3^3+...+3^{2019})`
`⇒S` chia `13` dư `12`
`⇒S` không chia hết `13`
`\text{©CBT}`