设等差数列[an]的前n项和为Sn,若a5=5a3,则S9/S5等于多少?
问题描述:
设等差数列[an]的前n项和为Sn,若a5=5a3,则S9/S5等于多少?
答
S9/S5
=2S9/2S5
=(a1+a9)*9/[(a1+a5)*5]
=9a5/5a3
=45a3/5a3
=9