\(S=\left(5+5^2+5^3+5^4\right)+...+\left(5^{93}+5^{94}+5^{95}+5^{96}\right).\)
\(S=5\left(1+5+5^2+5^3\right)+...+5^{93}\left(1+5+5^2+5^3\right)\)
\(S=156\left(5+5^5+5^9+...+5^{89}+5^{93}\right)\) chia hết cho 156
chứng minh chia hết cho 780 là chia hết cho 156