`(x - 1/5)^{2} = 4/25`
$\Rightarrow$ `(x - 1/5)^{2} = (+- 2/5)^{2}`
$\Rightarrow$ `x - 1/5 = +- 2/5`
$\Rightarrow$ $\left[\begin{matrix} x-\dfrac{1}{5}=\dfrac{2}{5}\\ x-\dfrac{1}{5}=\dfrac{-2}{5}\end{matrix}\right.$
$\Rightarrow$ $\left[\begin{matrix} x=\dfrac{2}{5}+\dfrac{1}{5}\\ x=\dfrac{-2}{5}+\dfrac{1}{5}\end{matrix}\right.$
$\Rightarrow$ $\left[\begin{matrix} x=\dfrac{3}{5}\\ x=\dfrac{-1}{5}\end{matrix}\right.$
$\text{Vậy}$ `x \in {3/5 ; -1/5}`