a) $|x-1| +3x=1$
$\to |x-1| = 1-3x$ $(ĐK : x ≤ \dfrac{1}{3}$ )
$\to \left[ \begin{array}{l}x-1=1-3x\\x-1=3x-1\end{array} \right.$
$\to \left[ \begin{array}{l}x=\dfrac{1}{2}\\x=0\end{array} \right.$
$\to x=0$ ( Thỏa mãn )
b) $|2x-1| = |2x+3|$
$\to \left[ \begin{array}{l}2x-1=2x+3\\2x+3=1-2x\end{array} \right.$
$\to \left[ \begin{array}{l}-1=3\text{(Vô lí)}\\x=\dfrac{-1}{2}\end{array} \right.$
