设Sn是等差数列{an}的前n项和,若a7a4=2,则S13S7的值为_.
问题描述:
设Sn是等差数列{an}的前n项和,若
=2,则a7 a4
的值为______. S13 S7
答
由题意可得
=S13 S7
=
13(a1+a13) 2
7(a1+a7) 2
13(a1+a13) 7(a1+a7)
=
=13×2a7
7×2a4
×13 7
=a7 a4
×2=13 7
.26 7
故答案为:
26 7