Đáp án+Giải thích các bước giải:
`(x+721)/2020+(x+21)/700+(x+721)/2021=-1`
`→(x+721)/2020+(x+21)/700+(x+721)/2021+1=0`
`→(x+721)/2020+((x+21)/700+1)+(x+721)/2021=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`
`→x+721=0(` vì `1/2020+1/700+1/2021\ne0``)`
`→x=-721`
Vậy `x=-721`
