\(M=2+2^2+2^3+...+2^{20}\)
\(\)\(M=\left(2+2^2\right)+\left(2^3+2^4\right)+...+\left(2^{19}+2^{20}\right)\)
\(M=5+2^2.\left(2+2^2\right)+...+2^{18}.\left(2+2^2\right)\)
\(M=5+2^2.\left(2+2^2\right)+...+2^{18}.\left(2+2^2\right)\)
\(M=5+2^2.5+...+2^{18}.5\)
\(M=5.\left(1+2^2+...+2^{18}\right)\)
\(\text{Do }5⋮5\Rightarrow5.\left(1+2^2+...+2^{18}\right)\)
\(\text{Hay A⋮5}\left(đpcm\right)\)
\(\text{Vậy }A⋮5\)