问题 选择题

若{an}是等差数列,则a6<a7是a6<a8的(  )

A.充分而不必要条件

B.必要而不充分条件

C.充分必要条件

D.既不充分也不必要条件

答案

由题意,设等差数列的公差为d,则若a6<a7,则d>0,∴a6-a8=-2d<0,∴a6<a8∴a6<a7是a6<a8的充分条件;

若a6<a8,则d>0,∴a6-a7=-2d<0,∴a6<a7,∴a6<a7是a6<a8的必要条件;

∴a6<a7是a6<a8的充要条件;

故选C.

阅读理解

阅读理解。

Dear Rose,

     I like your programmes very much. I'm now writing to ask you something about American social

customs .My American friends want me to join them in their dinner party. I am very happy and have

decided to go, but I'm a little worried about it, too. The social customs in my country are very different

from here, so I'm afraid of making mistakes. Should I bring a present, such as sweets or flowers? Should

I arrive on time or a little late?

At the dinner table, how can I know which fork or knife to use? How can I let the family know that I'm

thankful for their kindness? 

                                                                                                                                    Wang Lin

Dear Wang Lin,

     It's a good idea to bring a small present when you go to a dinner party. Flowers are always nice, or

you may bring a bottle of wine if you know that your friends drink it. You should arrive on time or five to

ten minutes late. Don't get there early. If you're going to be more than fifteen minutes late, you should

call and tell them. Try to be free at the dinner table. If you don't know about choosing the right fork or

knife, just watch the other

people, and follow them. If you still have no idea of what to do, ask the person next to you. If you like

the food, say so. Of course, you'll thank them for the meal and for their kindness. It's also a good idea

to send a thank you card the day after. 

                                                                                                                                       Rose  

1. Wang Lin wants to _____ .

A. make friends with Rose

B. know some social customs for a dinner party

C. make her friends happy

2. Rose is possibly _____ .

A. a good housewife

B. a TV hostess (主持人)

C. a member of the dinner party

3. If you're asked to a dinner party but not sure about what to bring as a present, it's good to bring _____ .

A. some fruits

B. a bottle of wine

C. some flowers 

4. Which of the following is NOT proper(适当的)for joining in a dinner party?

A.Get there 15 minutes earlier.

B. Make yourself at home.

C. Follow others to use forks and knives. 

5. From the letters we've learned it's very _____ to know something about American customs.

A. difficult

B. friendly

C. helpful

问答题