Đáp án + Giải thích các bước giải:
`S=1/3+1/9+1/27+...+1/729=1/3+1/3^2+1/3^3+...+1/3^6`
`→3S=1+1/3+1/9+...+1/243`
`→3S=1+1/3+1/3^+...+1/3^5`
`→3S-1S=2S=(1+1/3+1/3^+...+1/3^5)-(1/3+1/3^2+1/3^3+...+1/3^6)`
`→2S=1-1/3^6`
`→2S=728/729`
`→S=728/729 : 2`
`→S=364/729`