Bài 26:

`a) Q = 3/(4 . 5) + 3/(5 . 6) + 3/(6 . 7) + ... + 3/(79 . 80)`

`Q = 3 . ( 1/(4 . 5) + 1/(5 . 6) + 1/(6 . 7) + ... + 1/(79 . 80) )`

`Q = 3 . ( 1/4 - 1/5 + 1/5 - 1/6 + 1/6 - 1/7 + ... + 1/79 - 1/80 )`

`Q = 3 . ( 1/4 - 1/80 )`

`Q = 3 . 19/80`

`Q = 57/80`

`b) E = 3/(2 . 5) + 3/(5 . 8) + 3/(8 . 11) + ... + 3/(32 . 35)`

`E = 1/2 - 1/5 + 1/5 - 1/8 + 1/8 - 1/11 + ... + 1/32 - 1/35`

`E = 1/2 - 1/35`

`E = 33/70`

$\\$

Bài 27:

`a) P = 1/(4 . 7) + 1/(7 . 10) + 1/(10 . 13) + ... + 1/(31 . 34)`

`P = 1/3 . ( 3/(4 . 7) + 3/(7 . 10) + 3/(10 . 13) + ... + 3/(31 . 34) )`

`P = 1/3 . ( 1/4 - 1/7 + 1/7 - 1/10 + 1/10 - 1/13 + ... + 1/31 - 1/34 )`

`P = 1/3 . ( 1/4 - 1/34 )`

`P = 1/3 . 15/68`

`P = 5/68`

`b) N = 2/(1 . 5) + 2/(5 . 9) + 2/(9 . 13) + ... + 2/(101 . 105)`

`N = 1/2 . ( 4/(1 . 5) + 4/(5 . 9) + 4/(9 . 13) + ... + 4/(101 . 105) )`

`N = 1/2 . ( 1 - 1/5 + 1/5 - 1/9 + 1/9 - 1/13 + ... + 1/101 - 1/105 )`

`N = 1/2 . ( 1 - 1/105 )`

`N = 1/2 . 104/105`

`N = 52/105`