giải giúp mình với.cảm ơn mn rất nhêiuf (ko phải làm phần a nhé

`a)`

`A = 1/6 + 1/12 + 1/20 + 1/30 + ... + 1/9900`

`A = 1/(2 . 3) + 1/(3 . 4) + 1/(4 . 5) + 1/(5 . 6) + ... + 1/(99 . 100)`

`A = 1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + 1/5 - 1/6 + ... + 1/99 - 1/100`

`A = 1/2 - 1/100`

`A = 49/100`

`b)`

`B = 1/15 + 1/35 + 1/63 + 1/99 + ... + 1/323`

`B = 1/(3 . 5) + 1/(5 . 7) + 1/(7 . 9) + ... + 1/(17 . 19)`

`B = (1/(3 . 5) + 1/(5 . 7) + 1/(7 . 9) + ... + 1/(17 . 19)) . 1/2`

`B = (1/3 - 1/5 + 1/5 - 1/7 + 1/7 - 1/9 + ... + 1/17 - 1/19) . 1/2`

`B = (1/3 - 1/19) . 1/2`

`B = 16/57 . 1/2`

`B = 8/57`

`c)`

`C = 1/3 + 1/3 + 1/10 + 1/15 + ... + 1/46 + 1/45`

`C = 2/6 + 2/12 + 2/20 + 2/30 + ... + 2/72 + 2/90)`

`C = (1/6 + 1/12 + 1/20 + 1/30 + ... + 1/72 + 1/90) : 1/2`

`C = (1/(2 . 3) + 1/(3 . 4) + 1/(4 . 5) + 1/(5 . 6) + ... + 1/(9 . 10)) : 1/2`

`C = (1/2 - 1/10) : 1/2`

`C = 2/5 : 1/2`

`C = 4/5`

`d)`

`D = 1/6 + 1/30 + 1/70 + 1/126 + ... + 1/966`

`D = 2/12 + 2/60 + 2/140 + 2/252 + ... + 2/1932`

`D = (1/(2 . 6) + 1/(6 . 10) + 1/(10 . 14) + 1/(14 . 18) + ... + 1/(42 . 46)) . 2 . 4`

`D = (1/2 - 1/6 + 1/6 - 1/10 + 1/10 - 1/14 + 1/14 - 1/18 + ... + 1/42 - 1/46) : 8`

`D = (1/2 - 1/46) . 8`

`D = 11/23 . 8`

`D = 88/23`