1S4的精选

问题详情：设等比数列{an}的前n项和为Sn,S2=-1,S4=-5,则S6=()A.-9             B.-21         C.-25         D.-63【回答】B解析由题意,S2=a1+a2=-1,S4-S2=a3+a4=(a1+a2)q2=-4,q2=4,S6...