Question

\(Cho\) \(s=5+5^2\)\(+5^3\)\(+...+5^{2016}\)

Tính S

Answer 1

\(5S=5^2+5^3+...+5^{2017}\)

=>\(4S=5^{2017}-5\)

hay \(S=\dfrac{5^{2017}-5}{4}\)

Answer 2

5S=5²+5³+5⁴+...+5²⁰¹⁷

5S-S=(5²+5³+5⁴+...+5²⁰¹⁷)-(5+5²+5³+...+5²⁰¹⁶)

4S=5²⁰¹⁷-5

S=\(\dfrac{5^{2017}-5}{4}\)