等比数列{an}中,公比为q,前n项和为Sn,若Sn+1,Sn,Sn+2成等差则q=?

等比数列{an}中,公比为q,前n项和为Sn,若Sn+1,Sn,Sn+2成等差则q=?
等比数列{an}中,公比为q,前n项和为Sn,若Sn+1,Sn,Sn+2成等差则q=?

等比数列{an}中,公比为q,前n项和为Sn,若Sn+1,Sn,Sn+2成等差则q=?
Sn+1,Sn,Sn+2成等差则Sn+2-Sn=Sn-Sn+1
即an+2+an+1=-an+1即-2an+1=an+2.an+2=an+1*q=-2an+1
q=-2