a/ 2x - 1/4 = 0
<=> 2x = 1/4
<=> x = 1/8
Vậy: S = {1/8}
b/ 25x^2 - 10x = 0
<=> 5x ( 5x - 2 ) = 0
\(\Leftrightarrow\orbr{\begin{cases}5x=0\\5x-2=0\end{cases}\Leftrightarrow\orbr{\begin{cases}x=0\\5x=2\end{cases}\Leftrightarrow}\orbr{\begin{cases}x=0\\x=\frac{2}{5}\end{cases}}}\)
Vậy: S = {0;2/5}