设等比数列{an}的公比q=1/2,前n项的和为Sn,则S4/a4=?
问题描述:
设等比数列{an}的公比q=1/2,前n项的和为Sn,则S4/a4=?
答
S4=a1(1-1/(2^4)/(1-1/2)
=15a1/8
a4=a1/8
S4/a4=15