Question

高一数学题（关于等比数列）

在等比数列{An}中，若Sn=93，An=48，q=2，求n

Answer 1

A(n) = a2^(n-1), n = 1,2,...
S(n) = a[2^n - 1], n = 1,2,...

48 = a2^(n-1),
93 = a[2^n-1]=a2^n - a = 2[a2^(n-1)] - a = 2*48 - a = 96 - a, a = 3.
48 = a2^(n-1)=3*2^(n-1),
16 = 2^(n-1),
n = 5.

Answer 2

第一项为A1
Sn=A1（2^n-1）=93……①
An=A1×2^（n-1）=48……②
联立①②
得：n=5

Answer 3

5