A=4+$2^{2}$+$2^{3}$+$2^{4}$ +...+$2^{20}$
⇒2A=2(4+$2^{2}$+$2^{3}$+$2^{4}$ +...+$2^{20}$ )
⇒2A=8+$2^{3}$+$2^{4}$+$2^{5}$ +...+$2^{21}$
⇒2A-A=(8+$2^{3}$+$2^{4}$+$2^{5}$ +...+$2^{21}$)-(4+$2^{2}$+$2^{3}$+$2^{4}$ +...+$2^{20}$)
⇒2A-A=8+$2^{3}$+$2^{4}$+$2^{5}$ +...+$2^{21}$-4-$2^{2}$-$2^{3}$-$2^{4}$ -...-$2^{20}$
⇒2A-A=8+$2^{21}$-4-$2^{2}$
⇒A=$2^{21}$-(-8+4+$2^{2}$)
⇒A=$2^{21}$
⇒A=2097152
Vậy A=2097152
