\(A=4+2^3+2^4+2^5+...+2^{20}\)
\(A=2^2+2^3+2^4+2^5+...+2^{20}\)
\(\Rightarrow2A=2^3+2^4+2^5+2^6+...+2^{21}\)
\(\Rightarrow2A-A=\left(2^3+2^4+2^5+2^6+...+2^{21}\right)-\left(2^2+2^3+2^4+2^5+...+2^{20}\right)\)
\(\Rightarrow A=2^{21}-2^2\)
\(=2^2\left(2^{19}-1\right)\)
Vậy A là một lũy thừa của 2.
#kễnh