∵{an}为等比数列
∴a5-a1=a1(q^4-1)=15……①
a4-a2=a1(q³-q)=6……②
①/②式得：(q^4-1)/(q³-q)=15/6
化简得：2q²-5q+2=0
(q-2)(2q-1)=0
解得：q=2或1/2
1°当q=2时,a1=15/(q^4-1)=1
则a3=a1q²=1*2²=4
S8=a1(1-q^8)/(1-q)=1*(1-2^8)/(1-2)=255
2°当q=1/2时,a1=15/(q^4-1)=-16
则a3=a1q²=-16*(1/2)²=-1
S8=a1(1-q^8)/(1-q)=-16*[1-(1/2)^8]/(1-1/2)=-255/8