Ta có: \(A=5+4^2+4^3+\cdots+4^{2020}+4^{2021}\)
=>\(A=1+4+4^2+4^3+\cdots+4^{2020}+4^{2021}\)
=>\(4A=4+4^2+4^3+\cdots+4^{2021}+4^{2022}\)
=>\(4A-A=4+4^2+\cdots+4^{2022}-1-4-\cdots-4^{2021}\)
=>\(3A=4^{2022}-1\)
=>\(3A+1=4^{2022}=4^{2021}\cdot4\) ⋮\(4^{2021}\)