Đáp án + Giải thích các bước giải:
`B=1/120 - 2/30.33 - 2/33.36 - ... - 2/117.120`
`B=1/120-2/3(3/30.33 + 3/33.36 +3/36.39 ... + 2/117.120)`
`B=1/120-2/3(1/30-1/33+1/33-1/36+1/36-1/39+...+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`
Vậy `B=(-1)/120`