`M=2+2^2+2^3+2^4+...+2^20`
`M=(2+2^2+2^3+2^4)+(2^5+2^6+2^7+2^8)+...+(2^17+2^18+2^19+2^20)`
`M=2(1 +2+2^2+2^3) +2^5(1+2+2^2+2^3)+...+2^17(1 +2+2^2+2^3)`
`M=2.15+2^5 .15+...+2^17 .15`
`M=15(2+2^5+...+2^17)`
- Ta có : `15 vdots 5`
`=> 15(2+2^5+...+2^17) vdots 5`
`=> M vdots 5`