Đáp án:
$\begin{array}{l}
A = {2^1} + {2^2} + {2^3} + ... + {2^{90}}\\
= \left( {{2^1} + {2^2} + {2^3}} \right) + \left( {{2^4} + {2^5} + {2^6}} \right) + ... + \left( {{2^{88}} + {2^{89}} + {2^{90}}} \right)\\
= 2\left( {1 + 2 + {2^2}} \right) + {2^4}.\left( {1 + 2 + {2^2}} \right) + ... + {2^{88}}\left( {1 + 2 + {2^2}} \right)\\
= 2.7 + {2^4}.7 + ... + {2^{88}}.7\\
= \left( {2 + {2^4} + {2^{88}}} \right).7 \vdots 7
\end{array}$
