Ta có :
\(M=5+5^2+5^3+...+5^{60}\)
\(\Leftrightarrow\)\(5M=5^2+5^3+5^4+...+5^{61}\)
\(\Leftrightarrow\)\(5M-M=\left(5^2+5^3+5^4+...+5^{61}\right)-\left(5+5^2+5^3+...+5^{60}\right)\)
\(\Leftrightarrow\)\(4M=5^{61}-5\)
\(\Leftrightarrow\)\(M=\frac{5^{61}-5}{4}\)
Vậy \(M=\frac{5^{61}-5}{4}\)