Đáp án:
`x=29`
Giải thích các bước giải:
`1/5.8+1/8.11+1/11.13+...+1/(x(x+3))=27/480`
`=>1/3(3/5.8+3/8.11+3/11.13+...+3/(x(x+3)))=27/480`
`=>1/3(1/5-1/8+1/8-1/11+1/11-1/13+...+1/x-1/(x+3))=27/480`
`=>1/3(1/5-1/(x+3))=27/480`
`=>1/5-1/(x+3)=27/480:1/3`
`=>1/5-1/(x+3)=27/480\ .\ 3`
`=>1/5-1/(x+3)=81/480`
`=>1/5-1/(x+3)=27/160`
`=>1/(x+3)=1/5-27/160`
`=>1/(x+3)=32/160-27/160`
`=>1/(x+3)=32/160-27/160`
`=>1/(x+3)=5/160`
`=>1/(x+3)=1/32`
`=>x+3=32`
`=>x=32-3`
`=>x=29`
Vậy `x=29`
