tính tổng : 1/2021 + 2/2021 + 3/2021 + … + 2020/2021

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1. ` 1/2021 + 2/2021 + 3/2021 + … + 2020/2021`

   `=(1+2+…+2020)/2021`

   `={(2020+1)×2020:2}/2021`

   `=(2021×1010)/2021`

   `=1010`

    

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2. `1/2021 + 2/2021 + 3/2021 + … + 2020/2021`

   `= (1+2+3+…+2020)/2021`

   `= {(2020+1).[(2020-1):(2-1)+1)]:2}/2021`

   `= {2021.[(2019:1+1)]:2}/2021`

   `= {2021.2020:2}/2021`

   `= {2021.2020. 1/2}/2021`

   `= 2020. 1/2 = 1010`

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