Cách khác : Ta có : $\overline{abcde}=\overline{ab}^{3}$ vì $10000\leq \overline{ab}^{3}<10000\Rightarrow 22\leq \overline{ab}<47$
$\overline{abcde}=\overline{ab}^{3}\Rightarrow \overline{cde}=\overline{ab}(\overline{ab}^{2}-1000) \Rightarrow \overline{ab}^{2}-1000=\frac{\overline{cde}}{\overline{ab}}\Rightarrow \overline{ab}^{2}=1000+\frac{\overline{cde}}{\overline{ab}}<1046$
Vì $\frac{\overline{cde}}{\overline{ab}}\leq \frac{999}{22}<46$
$\Rightarrow 1000<\overline{ab}^{2}<1046\Rightarrow \overline{ab}=32$
$\Rightarrow q.e.d$
- Vu Thuy Linh likes this