Tham khảo
`\frac{x+721}{2020}+\frac{x+21}{700}+\frac{x+721}{2021}=-1`
`⇒ \frac{x+721}{2020}+\frac{x+21}{700}+1+\frac{x+721}{2021}=-1+1`
`⇒\frac{x+721}{2020}+\frac{x+721}{700}+\frac{x+721}{2021}=0`
`⇒(x+721)(\frac{1}{2020}+\frac{1}{700}+\frac{1}{2021})=0`
`⇔x+721=0`(Do ` \frac{1}{2020}+\frac{1}{700}+\frac{1}{2021} \ne 0)`
`⇒x=-721`
`\text{©CBT}`