Sn是等差数列{an}的前n项和,S7=3(a3+a7),那么S9/a4=?
问题描述：
Sn是等差数列{an}的前n项和,S7=3(a3+a7),那么S9/a4=?
答
s7=(a1+a7)*7/2=3(a3+a7)
所以7(a1+a1+6d)=6(a1+2d+a1+6d)
所以a1=3d
s9/a4
=[(a1+a9)*9/2]/a4
=(a1+a1+8d)*9/[2(a1+3d)]
=14d*9/12d
=21/2