Đáp án:
\[Vay\,\,Min\,\,A = 57\,\,\,khi\,\,\frac{3}{4} \le x \le \frac{9}{7}.\]
Giải thích các bước giải:
\[\begin{array}{l}
A = 28\left( {\left| {x - \frac{3}{4}} \right| + \left| {x + \frac{9}{7}} \right|} \right) = 28\left( {\left| {\frac{3}{4} - x} \right| + \left| {x + \frac{9}{7}} \right|} \right) \ge 28\left| {\frac{3}{4} - x + x + \frac{9}{7}} \right| = 57\\
Dau\,\, = \,\,\,xay\,\,ra\, \Leftrightarrow \left( {\frac{3}{4} - x} \right)\left( {x + \frac{9}{7}} \right) \ge 0\\
\Leftrightarrow \frac{3}{4} \le x \le \frac{9}{7}.\\
Vay\,\,Min\,\,A = 57\,\,\,khi\,\,\frac{3}{4} \le x \le \frac{9}{7}.
\end{array}\]
