Đáp án:
Giải thích các bước giải:
`B=1/3+1/9+1/27+...+1/2187+1/6561`
`B=1/3+1/3^2+1/3^3+...+1/3^6+1/3^7`
`3B=3(1/3^1+1/3^2+1/3^3+...+1/3^6+1/3^7)`
`3B=1+1/3+1/3^2+...+1/3^5+1/3^6`
`3B-B=(1+1/3+1/3^2+...+1/3^5+1/3^6)-(1/3+1/3^2+1/3^3+...+1/3^6+1/3^7)`
`2B=1+1/3+1/3^2+...+1/3^5+1/3^6-1/3-1/3^2-1/3^3-...-1/3^6-1/3^7`
`2B=1-1/3^7`
`2B=(3^7+1)/3^7`
`B=((3^7+1)/3^7):2`