`(x + 721)/2020 + (x + 21)/700 + (x + 721)/2021 = -1`
`=> (x + 721)/2020 + (x + 21)/700 + (x + 721)/2021 + 1 = -1 + 1`
`=> (x + 721)/2020 + (x + 21)/700 + (x + 721)/2021 + 1 = 0`
`=> (x + 721)/2020 + (x + 21)/700 + (x + 721)/2021 + 700/700 = 0`
`=> (x + 721)/2020 + (x + 21 + 700)/700 + (x + 721)/2021 = 0`
`=> (x + 721)/2020 + (x + 721)/700 + (x + 721)/2021 = 0`
`=> (x + 721) . (1/2020 + 1/700 + 1/2021) = 0`
`=>` \(\left[ \begin{array}{l}x + 721 = 0\\\dfrac{1}{2020} + \dfrac{1}{700} + \dfrac{1}{2021} = 0\end{array} \right.\)
Mà `1/2020 + 1/700 + 1/2021 \ne 0`
`=> x + 721 = 0`
`=> x = 0 - 721`
`=> x = -721`
Vậy `x = -721`
