Đáp án:
Giải thích các bước giải:
`A` `=` `1/2+1/6+1/12+1/20+1/30` `+` `.....` `1/n`
`<=>` `A` `=` `1/(1.2)` `+` `1/(2.3)` `+` `1/(3.4)` `+` `1/(4.5)` `+` `1/(5.6)` `+` `.......` `1/(x.x+1)` `=` `49/50`
`A` `=` `1-1/2` `+1/2` `-1/3` `+1/3-1/4` `+1/4-1/5` `+1/5` `-1/6` `+` `1/6` `.......` `+` `1/x` `-` `1/(x+1)` `=` `49/50`
`A` `=` `1-0-0-0-0-0` `-......` `0` `-` `1/(x+1)` `=` `49/50`
`A` `=` `1-` `1/(x+1)` `=` `49/50`
`A` `=` `50/50` `-` `1/(x+1)` `=` `49/50`
`<=>` `x+1` `=` `50`
`n` `=` `x.(x+1)` `=` `49.50`
`n` `=` `2450`
Vậy `n` `=` `2450`
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