`A= 1 + 1/2 + 1/2^2 + 1/2^3 +…+1/2^2021` `1/2A= 1/2(1+1/2 + 1/2^2 + 1/2^3+…+1/2^2021)` `1/2 A= 1/2 + 1/2^2 + 1/2^3 +…+1/2^2022` `A- 1/2A= 1+ 1/2 + 1/2^2 + 1/2^3+…+1/2^2021 – 1/2 – 1/2^2 – 1/2^3 -…-1/2^2022` `1/2A= 1 – 1/2^2022` `A= (1-1/2^2022) : 1/2` `A= (1- 1/2^2022). 2` `A= 2- 1/2^2021` Vậy `A= 2-1/2^2021` Bình luận
`A= 1 + 1/2 + 1/2^2 + 1/2^3 +…+1/2^2021`
`1/2A= 1/2(1+1/2 + 1/2^2 + 1/2^3+…+1/2^2021)`
`1/2 A= 1/2 + 1/2^2 + 1/2^3 +…+1/2^2022`
`A- 1/2A= 1+ 1/2 + 1/2^2 + 1/2^3+…+1/2^2021 – 1/2 – 1/2^2 – 1/2^3 -…-1/2^2022`
`1/2A= 1 – 1/2^2022`
`A= (1-1/2^2022) : 1/2`
`A= (1- 1/2^2022). 2`
`A= 2- 1/2^2021`
Vậy `A= 2-1/2^2021`
Đáp án:
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