`M =1/{1.4} +1/{4.7} + 1/{7.10} +1/{10.13} + 1/{13.16} +1/{16.19} +1/{19.22}`
`=> 3M = 3/{1.4} +3/{4.7} + 3/{7.10} +3/{10.13} + 3/{13.16} +3/{16.19} +3/{19.22}`
`=> 3M = 1 -1/4 +1/4 -1/7 +1/7 -1/10 +1/10 -1/13 +1/13-1/16 +1/16-1/19 +1/19 -1/22`
`=> 3M = 1 - 1/22 = 21/22`
`=> M = 21/22 : 3 = 7/22`
Vậy `M = 7/22`