Question

已知公比q小于零的等比数列,s4=-20,s8=-1640.求an

Answer 1

设s=a1(1-q^n)/(1-q),

则s4=a1(1-q^4)/(1-q)=-20

s8=a1(1-q^8)/(1-q)=-1640

s4/s8=(1-q^4)/(1-q^8)=1/82

(1+q^4)=82

q=3,或q=-3

a1=-1/2 或a1=1,由题意可知q＜0,则q=-3,然后带入等比数列公式既可!希望采纳哦!