设Sn是等差数列{an}的前n项和，且S9-S4=40，则S13的值为（） A

搜题请搜索小程序“爱下问搜题”

问题选择题

设S_n是等差数列{a_n}的前n项和，且S₉-S₄=40，则S₁₃的值为（　　）

A．52

B．104

C．112

D．208

答案

由题意可得：S₉-S₄=a₅+a₆+a₇+a₈+a₉=40，

由等差数列的性质可得5a₇=40，解得a₇=8，

而S₁₃=

13(a1+a13)

13×2a7

=13a₇=104

故选B

阅读理解

小题1:lf you want to go out for lunch on Sunday, you can call the number

A．83386223 or 83338989

B．87679690 or 83386223

C．83338989 or 87872738

D．87872738 or 87796960小题2:If you want to buy a new dress, you can go to

A．69 Hong Kong Road

B．368 East Huanshi Road

C．5300 Shanghai Road

D．635 Grand Road小题3: is open for eleven and a half hours at weekends.

A．Full Hotel

B．Hongnan Restaurant

C．History Museum

D．Baihe Shopping Center小题4:On Sunday morning, the coffee shop in Full Hotel doesn’ t open until ___.

A．6:30 am

B．7 :00 am

C．7:30 am

D．8 :00 arn小题5:You can know when you visit History Museum.

A．the history of South China

B．the history of Guangdong' s native people

C．the history of Shanghai.

D．the history of Hong Kong

问答题简答题

第五十一回哪些细节表现宝玉怜爱女孩子？