Đặt A= 1/3+1/3^2+1/3^3+.........+1/3^2020
=> 3A= 3(1/3+1/3^2+1/3^3+.........+1/3^2020)
= 1+1/3+1/3^2+1/3^3+.........+1/3^2019
=> 3A- A= 1+1/3+1/3^2+1/3^3+.........+1/3^2019- 1/3-1/3^2-1/3^3-.........-1/3^2020
=> 2A= 1- 1/3^2020= (3^2020- 1)/ 3^2020
=> A= (3^2020- 1)/ 2. 3^2020