Giúp điiiiii

$\displaystyle A=5+5^{2} +5^{3} +...+5^{18}$
a, Ta có:
$\displaystyle  \begin{array}{{>{\displaystyle}l}}
5\vdots 5\\
5^{2} \vdots 5\\
...\\
5^{18} \vdots 5\\
\Rightarrow A\vdots 5
\end{array}$
b,
$\displaystyle  \begin{array}{{>{\displaystyle}l}}
A=\left( 5+5^{2}\right) +5^{2}\left( 5+5^{2}\right) +5^{4}\left( 5+5^{2}\right) +...+5^{16}\left( 5+5^{2}\right)\\
A=\left( 5+5^{2}\right)\left( 1+5^{2} +5^{4} +...+5^{16}\right)
\end{array}$
mà $\displaystyle 5+5^{2} =30\vdots 6$
$\displaystyle \Rightarrow A\vdots 6$
c,
$\displaystyle  \begin{array}{{>{\displaystyle}l}}
A=\left( 5+5^{2} +5^{3}\right) +5^{3}\left( 5+5^{2} +5^{3}\right) +...+5^{15}\left( 5+5^{2} +5^{3}\right)\\
A=\left( 5+5^{2} +5^{3}\right)\left( 1+5^{3} +5^{6} +...+5^{15}\right)\\
A=155\left( 1+5^{3} +5^{6} +...+5^{15}\right)
\end{array}$
mà $\displaystyle 155\vdots 31$
$\displaystyle \Rightarrow A\vdots 31$