则公比q等于什么?
等比数列的首项a1=-1,前n项和为Sn,若S10/S5=31/32,则公比q等于什么?
S10/S5 =[a1(1-q^10)/(1-q)]/[a1(1-q^5)/(1-q)] =1+q^5 =31/32 q=-1/2
前n和公式:Sn=a1(1-q^n)/(1-q),代10和5进去,相除就是31/32
答:已知等比数列{an}的首项a1>0,公比q>-1, bn=a(n+1)-ka(n+2) ==>a(n+1)-k[a(n+1)*q] ==>a(n+1)(1-kq...详情>>
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