$\\$
`M = 1+3+3^2+...+3^{2019} + 3^{2020}`
`-> M-1 = (1+3+3^2+...+3^{2019} + 3^{2020}) - 1`
`-> M-1 = 1+3+3^2+...+3^{2019} + 3^{2020}-1`
`-> M-1 = (1-1) +3+3^2+...+3^{2019} + 3^{2020}`
`-> M-1 = 3+3^2+...+3^{2019} + 3^{2020}`
`-> M-1 = (3+3^2) + ... + (3^{2019} + 3^{2020})`
`-> M-1 = 3 (1+3) + ... + 3^{2019} (1 + 3)`
`-> M-1 = 3 . 4 + ... + 3^{2019} . 4`
`-> M-1 = 4 . (3+...+3^{2019})`
Vì `4 \vdots 4`
`-> 4 . (3+...+3^{2019}) \vdots 4`
`-> M-1 \vdots 4` (Điều phải chứng minh)
