已知等差数列{an}的前n项和为Sn,a1+a5=1/2,且a9=20,则S11=A.260 B.220 C.130 D.110
问题描述:
已知等差数列{an}的前n项和为Sn,a1+a5=1/2,且a9=20,则S11=
A.260 B.220 C.130 D.110
答
已知等差数列{an}的前n项和为Sn,a1+a5=1/2,且a9=20,则S11=
A.260 B.220 C.130 D.110