A= 4 +$2^{2}$ + $2^{3}$ + $2^{4}$ + .....+ $2^{20}$
A= 4 + ( $2^{2}$ + $2^{3}$ + $2^{4}$ + .....+ $2^{20}$ )
A-4 = $2^{2}$ + $2^{3}$ + $2^{4}$ + .....+ $2^{20}$
2 ( A-4 ) = $2^{3}$ + $2^{4}$ + $2^{5}$ + .....+ $2^{21}$
A-4 = 2 ( A-4 ) - A-4 =( $2^{3}$ + $2^{4}$ + $2^{5}$ + .....+ $2^{21}$)- ($2^{2}$ + $2^{3}$ + $2^{4}$ + .....+ $2^{20}$)
A-4= $2^{21}$ - 2²
A-4= $2^{21}$ - 4
A= $2^{21}$ - 4 + 4
A = $2^{21}$
