$( x+ \frac{1}{2} )^2 = \frac{4}{25}$
<=> $ \sqrt{ (x + \frac{1}{2} )^2 } = \sqrt{ \frac{4}{25} }$
<=> $ | x+ \frac{1}{2}| = \frac{2}{5}$
* TH1: $ x+ \frac{1}{2} = \frac{2}{5}$
<=> $ x= \frac{-1}{10}$
* TH2: $ x+ \frac{1}{2} = \frac{-2}{5}$
<=> $ x= \frac{-9}{10}$
Vậy $ S= ( \frac{-1}{10}; \frac{-9}{10} )$