等比数列前N项和得题目!已知a3=3/2,s3=9/2,求a1与q

Question

等比数列前N项和得题目!
已知a3=3/2 ,s3=9/2 ,求a1与q

Answer 1

a3=a1q^2=3/2,S3=a1+a2+a3=a1(1+q+q^2)=9/2,两式一除：(1+q+q^2)/q^2=3
解得：q=1或-1/2
当q=1时,a1=3/2
当q=-1/2时,a1=6.