Đáp án:
`x=305`
Giải thích các bước giải:
`1/20+1/44+1/77+...+2/(x(x+3))=101/770`
`=>1/20+1/44+1/77+...+1/(x(x+3)÷2)=101/770`
`=>1/40+1/88+1/154+...+1/(x(x+3))=101/1540`
`=>1/5.8+1/8.11+1/11.14+...+1/(x(x+3))=101/1540`
`=>1/3(3/5.8+3/8.11+3/11.14+...+3/(x(x+3)))=101/1540`
`=>1/3(1/5-1/8+1/8-1/11+1/11-1/14+...+1/x-1/(x+3))=101/1540`
`=>1/3(1/5-1/(x+3))=101/1540`
`=>1/5-1/(x+3)=101/1540÷1/3`
`=>1/5-1/(x+3)=303/1540`
`=>1/(x+3)=1/5-303/1540`
`=>1/(x+3)=1/308`
`=>x+3=308`
`=>x=308-3`
`=>x=305`
Vậy `x=305`.
