\(A=2+2^2+2^3+2^4+..+2^{20}\)
\(A=\left(2+2^2+2^3+2^4\right)+...+\left(2^{17}+2^{18}+2^{19}+2^{20}\right)\)
\(A=\left(...0\right)+...+\left(2^{4\cdot4+1}+2^{4\cdot4+2}+2^{4\cdot4+3}+2^{5\cdot4}\right)\)
\(A=\left(...0\right)+...+\left(2^{4\cdot4}\cdot2+2^{4\cdot4}\cdot2^2+2^{4\cdot4}\cdot2^3+2^{5\cdot4}\right)\)
\(A=\left(...0\right)+...+\left(...6\right)\cdot2+\left(...6\right)\cdot4+\left(...6\right)\cdot8+\left(...6\right)\)
\(A=\left(...0\right)+...+\left(...2\right)+\left(...4\right)+\left(...8\right)+\left(...6\right)\)
\(A=\left(...0\right)\)
Vậy chữ số tận cùng của A là 0
