Đáp án + Giải thích các bước giải:
`B = 1/120 - 2/30.33 - 2/33.36 - ... -2/117.120`
`B = 1/120 - (2/30.33 + 2/33.36 + ... + 2/117.120)`
`B = 1/20 - [ 2/3 . ( 1/30 - 1/33 + 1/33 - 1/36 + ... + 1/117 - 1/120)]`
`B = 1/120 - [ 2/3 . (1/30 - 1/120) ]`
`B = 1/120 - [ 2/3 . ( 4/120 - 1/120)]`
`B = 1/120 - [ 2/3. 1/40]`
`B = 1/120 - 1/60`
`B = 1/120 - 2/120`
`B = (-1)/120`