` (x+721) /2020+(x+21) /700+ (x+721)/2021=-1`
`<=>(x+721)/2020+(x+21) /700+ 1+ (x+721)/2021=0`
`<=>(x+721)/2020+(x+721) /700+ (x+721)/2021=0 `
`<=>(x +721) (1/2020+1/700+1/2021 )= 0`
Vì `1/2020+1/700+1/2021>0`
`=>x+721 =0 <=>x=-721`
Vậy `S={-721}`
Ta có: \(\dfrac{x+721}{2020}+\dfrac{x+21}{700}+\dfrac{x+721}{2021}=-1\)
\(\Leftrightarrow\dfrac{x+721}{2020}+\dfrac{x+721}{700}+\dfrac{x+721}{2021}=0\)
\(\Leftrightarrow\left(x+721\right)\left(\dfrac{1}{2020}+\dfrac{1}{700}+\dfrac{1}{2021}\right)=0\)
mà \(\dfrac{1}{2020}+\dfrac{1}{700}+\dfrac{1}{2021}>0\)
nên x+721=0
hay x=-721
Vậy: S={-721}
\(\frac{x+721}{2020}+\frac{x+21}{700}+\frac{x+721}{2021}=-1\)
\(\Leftrightarrow\frac{x+721}{2020}+\frac{x+21}{700}+1+\frac{x+721}{2021}=0\)
\(\Leftrightarrow\frac{x+721}{2020}+\frac{x+21}{700}+\frac{x+721}{2021}=0\)
\(\Leftrightarrow\left(x+721\right)\left(\frac{1}{2020}+\frac{1}{700}+\frac{1}{2021}\right)=0\)
\(\text{Vì }\frac{x+721}{2020}+\frac{x+21}{700}+\frac{x+721}{2021}>0\)
\(\Rightarrow x+721=0\)
\(x\) \(=0-721=-721\)
