Tham khảo
`A=3^{2020}-3^{2019}+3^{2018}-3^{2017}+...+3^2-3+1`
`⇒3A=3^{2021}-3^{2020}+3^{2019}-3^{2018}+...+3^3-3^2+3`
`⇒3A+A=3^{2021}-3^{2020}+3^{2019}-3^{2018}+...+3^3-3^2+3-(3^{2020}-3^{2019}+3^{2018}-3^{2017}+...+3^2-3+1)`
`⇒4A=3^{2021}+1`
`⇒A=\frac{3^{2021}+1}{4}`