Đáp án:
$\begin{array}{l}
A = 3 + {3^2} + {3^3} + {3^4} + ... + {3^{25}}\\
= 3 + \left( {{3^2} + {3^3} + {3^4} + {3^5}} \right) + .... + \left( {{3^{22}} + {3^{23}} + {3^{24}} + {3^{25}}} \right)\\
= 3 + 3\left( {1 + 3 + {3^2} + {3^3}} \right) + ... + {3^{22}}\left( {1 + 3 + {3^2} + {3^3}} \right)\\
= 3 + 3.40 + ... + {3^{22}}.40\\
= 3 + 40\left( {3 + {3^6} + ... + {3^{22}}} \right)\\
Do:\,40\left( {3 + {3^6} + ... + {3^{22}}} \right) \vdots 40\\
\Rightarrow 40\left( {3 + {3^6} + ... + {3^{22}}} \right) + 3\,\,\,chia\,40\,\,dư\,\,3
\end{array}$
