1.S8-S4=q^4S4
即q^4=2
S12-S8=q^8S4
则S12=14
2.设an的公比为q
S2n-Sn=q^nSn
S3n-S2n=q^2nSn
显然,Sn,S2n-Sn,S3n-S2n为公比为q^n的等比数列