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问题 选择题
已知数列{an}的通项公式为an=(
4
9
)n-1-(
2
3
)n-1，则数列{an}（　　）
A．有最大项，没有最小项
B．有最小项，没有最大项
C．既有最大项又有最小项
D．既没有最大项也没有最小项
答案

an=(

4
9
)n-1-(
2
3
)n-1=[(
2
3
)n-1]2-(
2
3
)n-1

令(

2
3
)n-1=t，则t是区间（0，1]内的值，而an=t2-t=(t-
1
2
)2-
1
4
，

所以当n=1，即t=1时，an取最大值，使(

2
3
)n-1最接近
1
2
的n的值为数列{an}中的最小项，

所以该数列既有最大项又有最小项．

故选C．

阅读理解

1  It might understand more than you think． 

A dog named Rico recognizes the names of about 200 objects，say researchers in Germany．  2  Its word-1earning skills are as good as those of a parrot or chimpanzee．  

In one experiment，the researehers took all 200 items that Rico is supposed to know and divided them into 20 groups of 10 objects． 3 In the tests，Rico got 37 out of 40 commands right．As the dog couldn’t see anyone to get clues，the seientists believe Rico must understand the meanings of certain words．   

In another experiment，the scientists took one toy that Rico had never seen before and put it in a room with seven toys whose names the dog already knew． 4  .

The correct object was chosen in seven out of 10 tests．suggesting that the dog had worked out the answer by process of removing．A month 1ater，Rico remembered half of the new names，which is even more impressive．

5 Even if they do，they call’t talk back．Still．it wouldn’t hurt to sweet-talk your dog every now and then．You might just get a big，wet kiss in return!

A．The owner then told Rico to fetch the object，using a word the dog had never heard before．

B．Do dogs more understand us than some friends?

C．It’s hard to know if all dogs understand at least some of the words we say．

D．Be careful of what you say around your dog． E．The dog also appears to be able to learn new words．

F．Then the owner told the dog to go and fetch one of the items．

G．Rico is thought to be smarter than the average dog．
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