Question

chưng tỏ 5^1 + 5^2+ ...+5^2005 = ?

giúp tui

Answer 1

\(A=5+5^2+...+5^{2005}\) 

\(5A=5^2+5^3+...+5^{2006}\) 

\(5A-A=\)\(\left(5^2+5^3+...+5^{2006}\right)-\left(5+5^2+...+5^{2005}\right)\) 

\(A=\frac{5^{2006}-5}{4}\)