Tính tổng : 2^1 + 2^2 + 2^3 + 2^4 + 2^5 + …. + 2^20

Question

Tính tổng : 2^1 + 2^2 + 2^3 + 2^4 + 2^5 + …. + 2^20

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Amity 1 year 2020-11-29T10:05:43+00:00 2 Answers 45 views 1

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    0
    2020-11-29T10:07:20+00:00

    Gọi A = `2^1` + `2^2` + `2^3` + `2^4` + `2^5` + …. + `2^20`

    2A = `2^2` + `2^3` + `2^4` + `2^5` + …. + `2^21`

    2A – A = (`2^2` + `2^3` + `2^4` + `2^5` + …. + `2^21`) – (`2^1` + `2^2` + `2^3` + `2^4` + `2^5` + …. + `2^20`)

    A = `2^21` – `2^1`

    A = `2^21` – 2

    XIN HAY NHẤT NHA

     

    0
    2020-11-29T10:07:26+00:00

    Đặt A=2^1+2^2+2^3+…..+2^20

    Ta có: A=2^1+2^2+2^3+…+2^20

    =>2A=2(2^1+2^2+2^3+…+2^20)=2^2+2^3+2^4+…+2^21

    =>A=2A-A=(2^2+2^3+2^4+…+2^21)-(2^1+2^2+2^3+…+2^20)

    =>A=2^21-2

     

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