Đáp án:
$\begin{array}{l}
B = 4 + {2^3} + {2^4} + ... + {2^{20}}\\
B = {2^2} + {2^3} + {2^4} + .. + {2^{20}}\\
\Rightarrow 2B = {2^3} + {2^4} + {2^5} + ... + {2^{20}} + {2^{21}}\\
\Rightarrow 2B - B = \left( {{2^3} + {2^4} + {2^5} + ... + {2^{20}} + {2^{21}}} \right) - \left( {{2^2} + {2^3} + {2^4} + .. + {2^{20}}} \right)\\
\Rightarrow B = {2^{21}} - {2^2} = {2^2}\left( {{2^{19}} - 1} \right)
\end{array}$
