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