Đáp án:
$\begin{array}{l}
2)\\
\overline {a0b} = \overline {7ab} \\
\Rightarrow 100a + b = 700 + 10a + b\\
\Rightarrow 90a = 700\\
\Rightarrow a = \dfrac{{70}}{9}\left( {ktm:do:a \in N*} \right)\\
\Rightarrow \text{không}\,\text{có}\,\overline {ab} \,\text{thỏa}\,\text{mãn}\\
3)a){81^5} = {\left( {{3^4}} \right)^5} = {3^{20}}\\
{27^7} = {\left( {{3^3}} \right)^7} = {3^{21}} > {3^{20}}\\
\Rightarrow {81^5} < {27^7}\\
b){5^{30}} = {\left( {{5^3}} \right)^{10}} = {125^{10}} > {123^{10}}\\
\text{Vậy}\,{5^{30}} > {123^{10}}\\
4)\\
\overline {21xy} \vdots 5\\
\Rightarrow \left[ \begin{array}{l}
y = 0\\
y = 5
\end{array} \right.\\
Do:\overline {21xy} \vdots 4\\
\Rightarrow y\,\text{chẵn}\\
\Rightarrow y = 0\\
\Rightarrow \overline {21xy} = \overline {21x0} \\
DO:\overline {21x0} \vdots 3\\
\Rightarrow 2 + 1 + x + 0 \vdots 3\\
\Rightarrow x \vdots 3\\
\Rightarrow \left[ \begin{array}{l}
x = 0\\
x = 3\\
x = 6\\
x = 9
\end{array} \right.\\
\text{Vậy}\,\overline {21xy} = \left\{ {2100;2130;2160;2190} \right\}
\end{array}$
