Question

登愣愣

想問問第13題 非常感謝！！

13.等比數列〈an>的前n項和為 Sn,且 Se是 S3的9倍,試問此數列的公比?
12

Answer 1

綾

Sn = a1(r^n - 1)/(r - 1)
S6 = a1(r^6 - 1)/(r - 1)
S3 = a1(r^3 - 1)/(r - 1)
(r^6 - 1 = (r^3 - 1)(r^3 + 1))
S6/S3 = r^3 + 1 = 9
r^3 = 8
r = 2