*Lời giải :
`M = 2/(7 . 9) + 3/(9 . 12) + 4/(12 . 16) + 5/(16 . 21)`
`⇔ M = 1/(7 . 9) + 1/(9 . 12) + 1/(12 . 16) + 1/(16 . 21)`
`⇔ M = 1/7 - 1/9 + 1/9 - 1/12 + 1/12 - 1/16 + 1/16 - 1/21`
`⇔ M = 1/7 + (- 1/9 + 1/9 - 1/12 + 1/12 - 1/16 + 1/16) - 1/21`
`⇔ M = 1/7 - 1/21`
`⇔ M = 2/21`
Vậy `M = 2/21`