Question

tính tổng 5+5^2+5^3+...+5^2006

Answer 1

Dat \(S=5+5^2+5^3+...+5^{2006}\)

\(\Rightarrow5S=5^2+5^3+5^4+...+5^{2007}\)

\(5S-S=\left(5^2+5^3+...+5^{2007}\right)-\left(5+5^2+...+5^{2006}\right)\)

\(4S=5^{2007}-5\)

\(\Rightarrow S=\frac{5^{2007}-5}{4}\)

Vay...