cứu với mn ơi ;-;; huhuuuuuu

`a, A = 1/(1.2) + 1/(2.3) + ... + 1/(99.100)`

`= 1/1 - 1/2 + 1/2 - 1/3 + ... + 1/99- 1/100`

`= 1 - 1/100`

`= 99/100`

Vậy `A = 99/100`

`b, B = (1 - 1/2)(1 - 1/3) ... (1 - 1/(n + 1))`

`= 1/2 . 2/3 ... n/(n + 1)`

`= (1.2...n)/(2.3...(n + 1))`

`= 1/(n + 1)`

Vậy `B = 1/(n + 1)`

`c,C =   -66 . (1/2 - 1/3 + 1/11) + 124.(-37) + 63.(-124)`

`= -66. ( 33/66 - 22/66 + 6/66) - 124.(37 + 63)`

`= -66. 17/66 - 124.100`

`= -17 - 12400`

`= -12417`

Vậy `C = -12417`

`d, D = 7/4 . (33/12 + 3333/20 + 333333/303030 + 33333333/42424242)`

`= 7/4 . (33/12 + 33/20 + 33/30+ 33/42)`

`= 7/4 . 33 . (1/12 + 1/20 + 1/30 + 1/42)`

`= 231/4 . (1/(3.4) + 1/(4.5) + 1/(5.6) + 1/(6.7))`

`= 231/4 . (1/3 - 1/4 + 1/4 - 1/5+ 1/5 -1/6 + 1/6 - 1/7)`

`= 231/4 . (1/3 - 1/7)`

`= 231/4 . 4/21`

`= 11`

Vậy `D = 11`

`|__L__|`