B=1/120 – 2/30.33 – 2/33.36 – … – 2/117.120

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`