`3/(5 . 7) + 3/(7 . 9) + 3/(9 . 11) + ... + 3/(59 . 61)`
`= 3/2 . [2/(5 . 7) + 2/(7 . 9) + 2/(9 . 11) + ... + 2/(59 . 61)]`
`= 3/2 . (1/5 - 1/7 + 1/7 - 1/9 + 1/9 - 1/11 + ... + 1/59 - 1/61)`
`= 3/2 . (1/5 - 1/61)`
`= 3/2 . (61/305 - 5/305)`
`= 3/2 . 56/305 = 84/305`
Học tốt. Nocopy.