Đáp án:
`x=(-4)/19`
Giải thích các bước giải:
$\rm\#Japanese$
`(1/3.8+1/8.13+1/13.18+...+1/33.38)-x=31/114`
`=>(5/3.8+5/8.13+5/13.18+...+5/33.38)-5x=155/114`
`=>((8-3)/3.8+(13-8)/8.13+(18-13)/13.18+...+(38-33)/33.38)-5x=155/114`
`=>(1/3-1/8+1/8-1/13+1/13-1/18+...+1/33-1/38)-5x=155/114`
`=>(1/3-138)-5x=155/114`
`=>35/114-5x=155/114`
`=>5x=(-20)/19`
`=>x=(-4)/19`
Vậy `x=(-4)/19`.