公比q=1时,S10/S5=10a1/5a1=2,与已知不符,因此q≠1.(这一步一定要判断一下)
S10/S5=[a1(q^10-1)/(q-1)]/[a1(q^5-1)/(q-1)]
=(q^10-1)/(q^5-1)
=(q^5-1)(q^5+1)/(q^5-1)
=q^5+1=31/32
q^5=-1/32
q=-1/2
a2=a1q=-1×(-1/2)=1/2
an=a1q^(n-1)=-(-1/2)^(n-1)