`M = 7/6 + 7/12 + 7/20 + ... + 7/1980`

`M = 7(1/6 + 1/12 + 1/20 + ... + 1/1980)`

`M = 7(1/(2 . 3) + 1/(3 . 4) + 1/(4 . 5) + ... + 1/(44 . 45))`

`M = 7(1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + ... + 1/44 - 1/45)`

`M = 7(1/2 - 1/45)`

`M = 7(45/90 - 2/90)`

`M = 7 . 43/90`

`M = (7 . 43)/90`

`M = 301/90`

        Vậy: `M = 301/90`

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