Đáp án:
$\begin{array}{l}
c)289 \le 9x \le 412\\
\Rightarrow \frac{{289}}{9} \le x \le \frac{{412}}{9}\\
\Rightarrow 32,11 \le x \le 45,77\\
\Rightarrow x \in \left\{ {33;34;...;44;45} \right\}\\
Vậy\,x \in \left\{ {33;34;...;44;45} \right\}\\
B3)\\
\overline {342**} \vdots 90\\
\Rightarrow \overline {342**} = \overline {342*0} \\
Do:\overline {342*0} \vdots 9\\
\Rightarrow 3 + 4 + 2 + * + 0 \vdots 9\\
\Rightarrow 9 + * \vdots 9\\
\Rightarrow \left[ \begin{array}{l}
* = 0\\
* = 9
\end{array} \right.\\
\Rightarrow \left[ \begin{array}{l}
\overline {342**} = \overline {34200} \\
\overline {342**} = \overline {34290}
\end{array} \right.
\end{array}$
