Đáp án:
`x=16`
Giải thích các bước giải:
`1/1.4+1/4.7+1/7.10+...+1/(x(x+3))=6/19`
`=>1/3[3/1.4+3/4.7+3/7.10+...+3/(x(x+3))]=6/19`
`=>1/3[1/1-1/4+1/4-1/7+1/7-1/10+...+1/x-1/(x+3)]=6/19`
`=>1/3[1/1-1/(x+3)]=6/19`
`=>1/1-1/(x+3)=6/19:1/3`
`=>1/1-1/(x+3)=6/19. 3/1`
`=>1/1-1/(x+3)=18/19`
`=>1/(x+3)=1/1-18/19`
`=>1/(x+3)=19/19-18/19`
`=>1/(x+3)=1/19`
`=>x+3=19`
`=>x=19-3`
`=>x=16`
Vậy `x=16`