tính tổng A=3mũ 2020 -3 mũ 2019+3 mũ 2018- 3 mũ 2017+…….+3 mũ2 -3 +1

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1. Tham khảo

    `A=3^{2020}-3^{2019}+3^{2018}-3^{2017}+…+3^2-3+1`

   `⇒3A=3^{2021}-3^{2020}+3^{2019}-3^{2018}+…+3^3-3^2+3`

   `⇒3A+A=3^{2021}-3^{2020}+3^{2019}-3^{2018}+…+3^3-3^2+3-(3^{2020}-3^{2019}+3^{2018}-3^{2017}+…+3^2-3+1)`

   `⇒4A=3^{2021}+1`

   `⇒A=\frac{3^{2021}+1}{4}`

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2. `A=3^2020 -3^2019+3^2018- 3^2017+…….+3^2 -3 +1`

   `3A = 3^2021 -3^2020+3^2019- 3^2018+…….+3^3 -3^2 +3`

   `3A+A=( 3^2021 -3^2020+3^2019- 3^2018+…….+3^3 -3^2 +3)`+`(3^2020 -3^2019+3^2018- 3^2017+…….+3^2 -3 +1)`

   `4A = 3^2021 + 1`

   `A = (3^2021 + 1)/4`

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