\(S=5+5^2+5^3+...+5^{2012}\)
\(\Rightarrow S=\left(5+5^2+5^3+5^4\right)+...+\left(5^{2009}+5^{2010}+5^{2011}+5^{2012}\right)\)
\(\Rightarrow S=\left(5+25+125+625\right)+...+5^{2009}\left(5+25+125+625\right)\)
\(\Rightarrow S=780+...+5^{2009}.780\)
\(\Rightarrow S=\left(1+...+5^{2009}\right).780⋮65\) ( 780 : 65 = 12 )
\(\Rightarrow S⋮65\left(đpcm\right)\)