Đáp án:
`11/12 + 11/(12 . 23) + ......... + 11/(89 . 100)`
` = 11/(1 . 12) + 11/(12 . 23) + ............. + 11/(89 . 100)`
` = 1 - 1/12 + 1/12 - 1/23 + .......... + 1/89 - 1/100`
` = 1 - (1/12 - 1/12) - (1/23 - 1/23) -............... - (1/89 - 1/89) - 1/100`
` = 1 - 1/100`
` = 99/100`