设sn是等差数列{an}的前n项和, 若a5:a7=2:3,求s9:(s13-s9)
问题描述:
设sn是等差数列{an}的前n项和, 若a5:a7=2:3,求s9:(s13-s9)
答
因为(s13-s9)/s9
=s13/s9-1
=13a7/9a5-1
=13*3/9*2-1
=21/18
=7/6
所以s9:(s13-s9)=6/7