Đáp án:

`a) A = 1/(8 . 11) + 1/(11 . 14) + .... + 1/(155 . 158)`

`A = 1/3 . (1/8 - 1/11 + 1/11 - 1/14 + .... + 1/155 - 1/158)`

`A = 1/3 . (1/8 - 1/158)`

`A = 1/3 . 75/632`

`A = 25/632`

Vậy `A = 25/632`

`b) B = 3/(13 . 19) + 3/(19 . 25) + ... + 3/(613 . 619)`

`B = 3/6 . (1/13 - 1/19 + 1/19 - 1/25 + ... + 1/613 - 1/619)`

`B = 1/2 . (1/13 - 1/619)`

`B = 1/2 . 606/8047`

`B = 303/8047`

Vậy `B = 303/8047`