Đáp án + Giải thích các bước giải:
`1/2 + 1/6 + 1/12 + 1/ 20 +...+1/(x(x+1))=2020/2021`
`=>1/1.2+1/2.3+1/3.4+1/4.5+...+1/(x(x+1))=2020/2021`
`=>1/1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+...+1/x-1/(x+1)=2020/2021`
`=>1/1-1/(x+1)=2020/2021`
`=>1/(x+1)=1/1-2020/2021`
`=>1/(x+1)=2021/2021-2020/2021`
`=>1/(x+1)=1/2021`
`=>x+1=2021`
`=>x=2021-1`
`=>x=2020`