题目内容
等差数列{an}前9项的和等于前4项的和.若ak+a4=0,则k=
A.8
B.9
C.10
D.11
等差数列{an}前9项的和等于前4项的和.若a1=1,ak+a4=0,则k=( )
A.10 B.9 C.8 D.7