Question

已知等比数列{an}中,S3=13/9,S6=364/9,求an

Answer 1

S6=a1(1-q^6)/(1-q)

S3=a1(1-q³)/(1-q)

S6/S3=(1-q^6)/(1-q³)=1+q³=28

q=3

所以13/9=a1(1-3³)/(1-3)

a1=1/9

an=3^(3-n)