tính nhanh : 2021×2020 – 1 / 2021×2019 + 2020

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1. $\frac{2021.2020-1}{2021.2019+2020}$ 

   = $\frac{2021.(2019+1)-1}{2021.2019+2020}$ 

   = $\frac{2021.2019+2021-1}{2021.2019+2020}$ 

   = $\frac{2021.2019+2020}{2021.2019+2020}$ 

   = 1

   Chúc học tốt!!!

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2.    $\dfrac{2021 × 2020 – 1}{2021 × 2019 + 2020}$

   = $\dfrac{2021 × (2019 + 1) – 1}{2021 × 2019 + 2020}$

   = $\dfrac{2021 × 2019 + (2021 – 1)}{2021 × 2019 + 2020}$

   = $\dfrac{2021 × 2019 + 2020}{2021 × 2019 + 2020}$

   = `1`

   $\text{Xin hay nhất}$

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