Đáp án:
Giải thích các bước giải:
a, Ta có:
\[\begin{array}{l}
\overline {ab} - \overline {ba} = 10a + b - \left( {10b + a} \right) = 10a + b - 10b - a = 9\left( {a + b} \right) \vdots 9\\
\Rightarrow \overline {ab} - \overline {ba} \vdots 9
\end{array}\]
b,
\[\begin{array}{l}
\overline {ab} = \overline {cd} \Rightarrow \overline {abcd} = \overline {abab} = \overline {ab} .100 + \overline {ab} = 101\overline {ab} \vdots 101\\
\Rightarrow \overline {abcd} \vdots 101
\end{array}\]
c,
\[\begin{array}{l}
\overline {abcd} = 100.\overline {ab} + \overline {cd} = 101\overline {ab} + \left( {\overline {cd} - \overline {ab} } \right)\\
101\overline {ab} \vdots 101\\
\overline {abcd} \vdots 101 \Rightarrow \overline {cd} - \overline {ab} \vdots 101\\
\Rightarrow \overline {ab} - \overline {cd} \vdots 101
\end{array}\]
