A = 2 + $2^{2}$ + $2^{3}$ + $2^{4}$ + ... + $2^{2020}$
2A = $2^{2}$ + $2^{3}$ + $2^{4}$ + ... + $2^{2021}$
2A - A = ($2^{2}$ + $2^{3}$ + $2^{4}$ + ... + $2^{2021}$) -
(2 + $2^{2}$ + $2^{3}$ + $2^{4}$ + ... + $2^{2020}$)
A = $2^{2021}$ - 2
Vì $2^{2021}$ - 2 < $2^{2021}$ nên A < B