\(\Leftrightarrow\left[{}\begin{matrix}3x-\dfrac{2}{5}=\dfrac{4}{5}\\3x-\dfrac{2}{5}=-\dfrac{4}{5}\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}3x=\dfrac{6}{5}\\3x=-\dfrac{2}{5}\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{2}{5}\\x=-\dfrac{2}{15}\end{matrix}\right.\)
\(\left(\dfrac{2}{5}-3x\right)^2=\left(\dfrac{-4}{5}\right)^2\)
\(\text{Vậy}\dfrac{2}{5}-3x=\left(\dfrac{-4}{5}\right)\)
\(3x=\dfrac{2}{5}+\dfrac{4}{5}=\dfrac{6}{5}\)
\(x=\dfrac{6}{5}:3=\dfrac{2}{5}\)
\(\text{hoặc}\dfrac{2}{5}-3x=\dfrac{4}{5}\)
\(3x=\dfrac{2}{5}+\left(\dfrac{-4}{5}\right)=\left(\dfrac{-2}{5}\right)\)
\(x=\dfrac{-2}{5}:3=\dfrac{-2}{15}\)
\(\Rightarrow x\in\left\{\dfrac{2}{5};\dfrac{-2}{15}\right\}\)
\(\text{Hok tốt!}\)