`M = 1/6 + 1/10 + 1/15 + 1/21 + 1/28 + 1/36 + 1/45 + 1/55`

`M = 2 xx ( 1/12 + 1/20 + 1/30 + 1/42 + 1/56 + 1/72 + 1/90 + 1/110 ) `

`M = 2 xx ( 1/( 3 xx 4 ) + 1/(4xx5) + 1/(5xx6) + 1/(6 xx 7 ) + 1/(7 xx 8) + 1/(8 xx 9 ) + 1/(9 xx 10 ) + 1/(10 xx 11 ) )`

`M = 2 xx ( 1/3 - 1/4 + 1/4 - 1/5 + 1/5 - 1/6 + 1/6 - 1/7 + 1/7 - 1/8 + 1/8 - 1/9 + 1/9 - 1/10 + 1/10 - 1/11 )`

`M = 2 xx ( 1/3 - 1/11 ) `

`M = 2 xx ( 11/33 - 3/33 ) `

`M = 2 xx 8/33`

`M = 16/33`

$Vậy$ `M = 16/33`

`@Nam`