\[\begin{array}{l}
A = \left| {x - \frac{4}{7}} \right| - \frac{1}{2}\\
Ta\,\,co:\,\,\left| {x - \frac{4}{7}} \right| \ge 0\,\,\forall x\\
\Rightarrow A \ge - \frac{1}{2}\,\,\forall x\\
Dau\,\, = \,\,xay\,\,ra \Leftrightarrow x - \frac{4}{7} = 0\\
\Leftrightarrow x = \frac{4}{7}.\\
Min\,\,A = - \frac{1}{2}\,\,khi\,\,x = \frac{4}{7}.
\end{array}\]
