设等比数列an的公比q=2,前n项和为Sn,则S4/a4=
问题描述:
设等比数列an的公比q=2,前n项和为Sn,则S4/a4=
答
设等比数列an的公比q=2,首项a1
A4=a1*q^3=8a1
S4=a1+a2+a3+a4=a1(1+q+q^2+q^3)=15a1
S4/a4=15/8