\(\begin{array}{l}
+ )\,\,\,\overline {23x4} \,\,\,chia\,\,\,het\,\,cho\,\,\,3\\
Ta\,\,\,co:\,\,\,2 + 3 + x + 4 = 9 + x\\
\Rightarrow \,\overline {23x4} \,\,\,chia\,\,\,het\,\,cho\,\,\,3 \Leftrightarrow \left( {9 + x} \right)\,\, \vdots \,\,3\\
\Rightarrow \left[ \begin{array}{l}
9 + x = 9\\
9 + x = 12\\
9 + x = 15\\
9 + x = 18
\end{array} \right. \Leftrightarrow \left[ \begin{array}{l}
x = 0\\
x = 3\\
x = 6\\
x = 9
\end{array} \right.\\
\Rightarrow x \in \left\{ {0;\,\,3;\,\,6;\,\,9} \right\}.\\
+ )\,\,\,\overline {563x} \,\,\,chia\,\,het\,\,\,cho\,\,\,9\\
Ta\,\,\,\,co:\,\,\,5 + 6 + 3 + x = 14 + x\,\,\\
\Rightarrow \overline {563x} \,\,\,chia\,\,het\,\,\,cho\,\,\,9 \Leftrightarrow \left( {14 + x} \right)\,\, \vdots \,\,9\\
\Leftrightarrow \left[ \begin{array}{l}
14 + x = 18\\
14 + x = 27
\end{array} \right. \Leftrightarrow \left[ \begin{array}{l}
x = 4\\
x = 13\,\,\,\left( {ktm} \right)
\end{array} \right.\\
Vay\,\,\,x = 4.\\
+ )\,\,\,\overline {73xy} \,\,\,chia\,\,\,het\,\,\,cho\,\,\,5\,\,\,\,va\,\,\,9\\
\Rightarrow \overline {73xy} \,\,\,chia\,\,\,het\,\,\,cho\,\,\,5\\
\Rightarrow y = 0\,\,hoac\,\,\,y = 5.\\
+ )\,\,\,Voi\,\,\,y = 5 \Rightarrow \overline {73x5} \\
co\,\,7 + 3 + x + 5 = x + 15 \Rightarrow \overline {73x5} \,\,\,chia\,\,\,het\,\,\,cho\,\,\,9\\
\Leftrightarrow \left[ \begin{array}{l}
x + 15 = 18\\
x + 15 = 27
\end{array} \right. \Leftrightarrow \left[ \begin{array}{l}
x = 3\\
x = 12\,\,\,\left( {ktm} \right)
\end{array} \right.\\
\Rightarrow x = 3.\\
+ )\,\,Voi\,\,y = 0 \Rightarrow \overline {73x5} \\
co\,\,7 + 3 + x + 0 = x + 10 \Rightarrow \overline {73x0} \,\,\,chia\,\,\,het\,\,\,cho\,\,\,9\\
\Leftrightarrow x + 10 = 18\\
\Leftrightarrow x = 8\\
\Rightarrow x = 8.\\
Vay\,\,so\,\,\,can\,\,tim:\,\,\,7380\,\,hoac\,\,\,7335.
\end{array}\)
Các câu còn lại bạn làm tương tự nhé!!!
