Đáp án+Giải thích các bước giải:
`1/3+1/6+1/10+1/15+....+1/(x(2x+1))=1/10`
`2 . [1/6+1/12+1/20+1/30+....+1/(2x(2x+1))]=1/10`
`2 . [1/(2 . 3) + 1/(3 . 4) + 1/(4 . 5) + ... + 1/(2x . (2x + 1))] = 1/10`
`2 . (1/2 - 1/3 + 1/3 -1/4 + 1/4 - 1/5 + ... + 1/(2x) - 1/(2x + 1)) = 1/10`
`2 . (1/2 - 1/(2x + 1)) = 1/10`
`1/2 - 1/(2x + 1) = 1/10 : 2`
`1/2 - 1/(2x + 1) = 1/20`
`1/(2x + 1) = 1/2 - 1/20`
`1/(2x + 1) = 9/20`
`2x + 1 = 1 : 9/20`
`2x + 1 = 20/9`
`2x = 11/9`
`x = 11/18`
Vậy `x = 11/18`
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