Thay $x=-1$ vào $A$, ta có:
$A=(-1)^{100}-2020.(-1)^{99}+2020.(-1)^{98}-2020.(-1)^{97}+.....-2020.(-1)+2020$
$=1-2020.(-1)+2020.1-2020.(-1)+.....-2020.(-1)+2020$
$=1+\underbrace{2020+2020+.....+2020}_{\text{$100$ số hạng}}$
$=1+2020.100$
$=1+202000$
$=202001$