数列{an}满足an+an+1= 12 （n∈N*），a2=2，Sn是数列{an}的前n项和，则S21为

题干

数列{a_n}满足a_n+a_n₊₁=

1

2

（n∈N^*），a₂=2，S_n是数列{a_n}的前n项和，则S₂₁为（）

A：5

B： $\frac{7}{2}$

C： $\frac{9}{2}$

D： $\frac{13}{2}$

上一题下一题 0.0难度选择题更新时间:2017-10-23 02:53:03

答案(点此获取答案解析）

B

同类题3

根据短文回答，完成作息时间表（时间都用英语表达）。

Peter plays basketball for the kids basketball club. He usually gets up at 5:30 and runs in his school from 5:40 to 6:40.After running, he has breakfast at 7 o'clock. For breakfast, he eats some eggs and vegetables. At 8:00,he plays basketball with his friends. They play for a long time. At about 12:00,they have lunch. For lunch, Peter has lots of vegetables, chicken and fruit. At 3 o'clock in the afternoon, Peter goes to the kids basketball club. Peter has dinner at about 7:00 in the evening. After dinner, he watches basketball games on TV. Peter usually goes to bed at 9:30.