Đáp án:
`S = 3/5`
Giải thích các bước giải:
`S = 1 - 11/6 + 19/12 - 29/20 + 41/30 - 55/42 + 71/56 - 89/72 + 109/90`
`= (-5)/6 + 19/12-29/20 + 41/30 - 55/42 + 71/56 - 89/72 + 109/90`
`= 3/4 - 29/20 + 41/30 - 55/42 + 71/56 - 89/72 + 109/90`
`= (-7)/10 + 41/30 - 55/42 + 71/56 - 89/72 + 109/90`
`= 2/3 - 55/42 + 71/56 - 89/72 + 109/90`
`= (-9)/14 + 71/56 - 89/72 + 109/90`
`= 5/8 - 89/72 + 109/90`
`= (-11)/18 + 109/90`
`= 3/5`