Question

已知等比数列中a3=3/2,S3=9/2,求a1和q?

Answer 1

S3=9/2

a3=3/2

a1+a2=6/2=3

a1(1+q)=3

a1q^2=3/2

(1+q)/q^2=2

1+q=2q^2

(2q+1)(q-1)=0

q=1或者q=-1/2

当q=1的时候,a1=3/2

当q=-1/2的时候

a1=a3/q^2=6

祝开心