Question

高一数学。大神帮忙

已知等比数列an的前n项和为Sn,且满足Sn=3^n＊k-二分之一.(1) 求k的值

Answer 1

Sn=3^n＊k-1/2
a1=S1=2k-1/2
n≥2时，
an=Sn-S(n-1)
=k*3^n-k*3^(n-1)
=2k*3^(n-1)
∴a2=6k,a3=18k
若{an}是等比数列
则渗运虚念a²2=a1*a3
∴丛誉梁36k²=(2k-1/2)*18k (k≠0)
解得k=1/2

Answer 2

a1=S1=3k-1/模凯2
a2=S2-a1=9k-1/旦闹唤2-(3k-1/2)=6k
a3=S3-S2=27k-9k=18k
a2^2=a1*a3
36k^2=18k(3k-1/2)
k1=0,k不弯迟=0时有2k=3k-1/2
k=1/2

Answer 3

s1=a1 求出看k值