Đáp án + Giải thích các bước giải:
`1/2+1/6+1/12+1/20+...+1/(x(x+1))=2021/2022`
`=>1/1.2+1/2.3+1/3.4+1/4.5+...+1/(x(x+1))=2021/2022`
`=>1/1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+...+1/x-1/(x+1)=2021/2022`
`=>1/1-1/(x+1)=2021/2022`
`=>1/(x+1)=1/1-2021/2022`
`=>1/(x+1)=2022/2022-2021/2022`
`=>1/(x+1)=1/2022`
`=>x+1=2022`
`=>x=2022-1`
`=>x=2021`