Question

tính 

s=5^2+5^3+.....+5^100

Answer 1

5s=5³+5⁴+............+5¹⁰¹

5s-s=(5³+5⁴+............+5¹⁰¹)-(5²+5³+...........+5¹⁰⁰)

=>4s=5¹⁰¹-5²

=>s=\(\frac{5^{101}-5^2}{4}\)

Answer 2

5s=5(5^3+5^4+.....+5^101)

s=(5^2+5^3+.....+100)

lấy 5s-s

4s=5^2-5^101