等比数列的前n项和为Sn,a1为3/2,且S3,S5,S4成等差数列,求an
问题描述：
等比数列的前n项和为Sn,a1为3/2,且S3,S5,S4成等差数列,求an
答
S5-S3=S4-S5
S3+a4+a5-S3=S5-a5-S5
a4+2a5=0
a4+2a4q=0
q=-1/2
an=a1q^(n-1)=3/2×(-1/2)^(n-1)=-3×(-1/2)^n