已知等差数列{an}的前n项和为Sn,a3+a8=13,且S7=35.则a7=( ) A.11 B.10 C.9 D.8
问题描述:
已知等差数列{an}的前n项和为Sn,a3+a8=13,且S7=35.则a7=( )
A. 11
B. 10
C. 9
D. 8
答
由等差数列的性质可得:
S7=
=7(a1+a7) 2
=35,解得a4=5,7×2a4
2
又a3+a8=a4+a7=13,故a7=8,
故选D