tính: $\frac{1}{120}$ – $\frac{2}{30.33}$ – $\frac{2}{33.36}$ – …- $\frac{2}{117.120}$

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1.  `\frac{1}{120} – \frac{2}{30.33} – \frac{2}{33.36} – … – \frac{2}{117.120}`

   `= \frac{1}{120} – [ \frac{2}{3} ( \frac{2}{30.33} – \frac{3}{33.36} – … – \frac{3}{117.120} )]`

   `= \frac{1}{120} – [ \frac{2}{3} ( \frac{1}{30} – \frac{1}{33} – \frac{1}{33} – \frac{1}{36} – … – \frac{1}{117} – \frac{1}{120} ) ]`

   `= \frac{1}{120} – [ \frac{2}{3} ( \frac{1}{30} + \frac{1}{120} ) ]`

   `= \frac{1}{120} – [ \frac{2}{3} . \frac{1}{24} ]`

   `= \frac{1}{120} – \frac{2}{72}`

   `= \frac{-168}{8640} = \frac{-7}{360}`

    

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