Đáp án:
Giải thích các bước giải:
`A` `=` `(-20)/(11.13)` `-` `20/(13.15)` `-` `20/(15.17)` `-` `20/(17.19)` `.....` `-20/(53.55)`
`A` `=` `(-20)/(11.13)` `+` `(-20)/(13.15)` `+` `(-20)/(15.17)` `+` `(-20)/(17.19)` `.....` `(-20)/(53.55)`
`-1/10` `A` `=` `2/(11.13)` `+` `2/(13.15)` `+` `2/(15.17)` `+` `(2)/(17.19)` `......` `2/(53.55`
`(-1)/10` `A` `=` `1/11` `-` `1/13` `+1/13` `-1/15` `+1/15-1/17` `+1/17` `-1/19` `.......` `1/53` `-` `1/55`
`(-1)/10` `A` `=` `1/11` `-` `1/55`
`(-1)/10` `A` `=` `5/11` `-` `1/55`
`(-1)/10` `A` `=` `4/55`
`A` `=` `4/55` `.` `(-10)`
`A` `=` `(-40)/55`
`A` `=` `(-8)/11`
Vậy `A` `=` `(-8)/11`
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