Ta có :\(A=5+5^2+5^3+...+5^{2016}\)
\(=\left(5+5^2+5^3\right)+\left(5^4+5^5+5^6\right)+...+\left(5^{2014}+5^{2015}+5^{2016}\right)\)
\(=5\left(1+5+5^2\right)+5^4\left(1+5+5^2\right)+...+5^{2014}\left(1+5+5^2\right)\)
\(=5.31+5^4.31+...+5^{2014}.31\)
\(=31\left(5+5^4+...+5^{2014}\right)⋮31\)
vậy A là bội của 31