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Đáp án:

Giải thích các bước giải:

 `A=1/3+1/3^3+1/3^3+...+1/3^20`

`3A=3*(1/3+1/3^3+1/3^3+...+1/3^20)`

`=1+1/3+1/3^3+1/3^3+...+1/3^20`

`3A-A=(1+1/3+1/3^3+1/3^3+...+1/3^21)`

`-(1/3+1/3^3+1/3^3+...+1/3^20)`

`2A=1-1/3^20`

`=>A=(1-1/3^20)/2`

`Vậy    A=(1-1/3^20)/2`

`B=(-1/3)^2-(-12/4)^6+(-1/2)^3*4`

`=1/3^2- 3^6-1/2^3*4`

`=1/9-729-1/8*4`

`=1/9-729-1/2`

`=1/9-1459/2`

`=2/18-13131/18`

`=-13129/18`

`240520122008212`