`A=1+3^2+3^3+.....+3^{2018}+3^{2019}`
Đặt `C=3^2+3^3+.....+3^{2018}+3^{2019}`
`3C=3^3+3^4+.....+3^{2019}+3^{2020}`
`3C-C=(3^3+3^4+.....+3^{2019}+3^{2020})-(3^2+3^3+.....+3^{2018}+3^{2019})`
`2C=3^{2020}-3^2`
`C=(3^{2020}-3^2)/2`
`A=C+1`
`<=>A=(3^{2020}-9)/2+1`
`<=>A=(3^{2020}-7)/2`
`=>B>A`