So sanh S với 3 biết : S = 2019/2020 + 2020/2021 + 2021/2019

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1. `S=2019/2020+2020/2021+2021/2019`

     `=(1-1/2020)+(1-1/2021)+(1+2/2019)`

     `=1-1/2020+1-1/2021+1+2/2019`

     `=1+1+1-1/2020-1/2021+2/2019`

     `=3-1/2020-1/2021+2/2019`

     `=3+(2/2019-1/2020-1/2021)`

   Vì: `1/2020;1/2021<1/2019`

   `⇒1/2019+1/2019>1/2020+1/2021`

   `⇒2/2019>1/2020+1/2021`

   `⇒2/2019-(1/2020+1/2021)>0`

   `⇒2/2019-1/2020-1/2021)>0`

   `⇒S>3+0`

   `⇒S>3`

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2. Đáp án+Giải thích các bước giải:

   `S-3=(2019/2020 +2020/2021 +2021/2019)-3`$\\$`=(2019/2020-1)+(2020/2021-1)+(2021/2019-1)`$\\$`=-1/2020-1/2021+2/2019`$\\$`=(1/2019-1/2020)+(1/2019-1/2021)>0`$\\$`=>2019/2020 +2020/2021 +2021/2019>3`

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