设等比数列an的前n项和为Sn,若S6/S2 =7,则S8/S4等于多少大神们帮帮忙

问题描述:

设等比数列an的前n项和为Sn,若S6/S2 =7,则S8/S4等于多少大神们帮帮忙

S6=a1(1+q)(1+q^2+q^4)=7a1(1+q) q^2+q^4=6 q^2=2 S8=S4(1+q^4) 所以S8/S4=5