由题意设数列{an}的首项为a1,32【课时精练】√1.已知Sn为数列{an}的前n项和,若S2=6,an+1=2an,则S100=A.252-4 B.252-2C.2100-2 D.2101-2因为an+1=2an,√2.记Sn为等比数列{an}的前n项和.若S2=4,S4=6,则S6=A.7 B.8 C.9 D.10因为S4=4,S4=6,易知公比q≠±1,√√4.若等比数列{an}的前n项和Sn=2n-1+a,则a3a5等于A.4 B.8 C.16 D.32等比数列{an}的前n项和Sn=2n-1+a,n≥2时,an=Sn-Sn-1=2n-1+a