Ta có: \(A=3^{2020}+3^{2019}+...+3^2+3\)
\(\Rightarrow3A=3^{2021}+3^{2020}+...+3^3+3^2\)
\(\Rightarrow3A-A=\left(3^{2021}+3^{2020}+...+3^2\right)-\left(3^{2020}+3^{2019}+...+3\right)\)
\(\Leftrightarrow2A=3^{2021}-3\)
\(\Rightarrow A=\frac{3^{2021}-3}{2}\)
Vậy \(A=\frac{3^{2021}-3}{2}\)