Giải thích các bước giải:
cách 1:
`E=1/3+1/3^2+1/3^3+...+1/3^2021`
`3E=1+1/3+1/3^2+...+1/3^2020`
`3E-E=(1+1/3+1/3^2+...+1/3^2020)-(1/3+1/3^2+1/3^3+...+1/3^2021)`
`2E=1-1/3^2021`
`2E=[3^2021-1]/3^2021`
`E=[3^2021-1]/3^2021:2`
`E=[3^2021-1]/3^2021. 1/2`
`E=[3^2021-1]/[3^2021 .2]`
cách 2:
`E=1/3+1/3^2+1/3^3+...+1/3^2021`
`3E=1+1/3+1/3^2+...+1/3^2020`
`3E-E=(1+1/3+1/3^2+...+1/3^2020)-(1/3+1/3^2+1/3^3+...+1/3^2021)`
`2E=1-1/3^2021`
`E=[1-1/3^2021]/2`
