Looking at the signs: $+ - - + - - + - - + \ldots$
The pattern repeats every 3 terms, so we group accordingly.
Group the terms:
$S = \left(1 - \dfrac{1}{2} - \dfrac{1}{4}\right) + \left(\dfrac{1}{8} - \dfrac{1}{16} - \dfrac{1}{32}\right) + \left(\dfrac{1}{64} - \dfrac{1}{128} - \dfrac{1}{256}\right) + \ldots$
Calculate each group:
Group 1: $1 - \dfrac{1}{2} - \dfrac{1}{4}$
Convert to same denominator:
$\dfrac{4}{4} - \dfrac{2}{4} - \dfrac{1}{4} = \dfrac{4-2-1}{4} = \dfrac{1}{4}$
Group 2: $\dfrac{1}{8} - \dfrac{1}{16} - \dfrac{1}{32}$
$\dfrac{4}{32} - \dfrac{2}{32} - \dfrac{1}{32} = \dfrac{4-2-1}{32} = \dfrac{1}{32}$
Group 3: $\dfrac{1}{64} - \dfrac{1}{128} - \dfrac{1}{256}$
$\dfrac{4}{256} - \dfrac{2}{256} - \dfrac{1}{256} = \dfrac{4-2-1}{256} = \dfrac{1}{256}$
Rewrite as a new series:
$S = \dfrac{1}{4} + \dfrac{1}{32} + \dfrac{1}{256} + \ldots$
Notice that:
$\dfrac{1}{32} = \dfrac{1}{4} \times \dfrac{1}{8}$
$\dfrac{1}{256} = \dfrac{1}{4} \times \dfrac{1}{64} = \dfrac{1}{4} \times \left(\dfrac{1}{8}\right)^2$
So:
$S = \dfrac{1}{4}\left(1 + \dfrac{1}{8} + \dfrac{1}{64} + \ldots\right)$
The series inside parentheses is geometric with:
- First term: $a = 1$
- Common ratio: $r = \dfrac{1}{8}$
Using the infinite geometric series formula $\dfrac{a}{1-r}$:
$\text{Sum inside parentheses} = \dfrac{1}{1-\dfrac{1}{8}} = \dfrac{1}{\dfrac{7}{8}} = \dfrac{8}{7}$
Final calculation:
$S = \dfrac{1}{4} \times \dfrac{8}{7} = \dfrac{8}{28} = \dfrac{2}{7}$