问题 解答题
在数列{an}中,an=
1
n+1
+
2
n+1
+…+
n
n+1
,又bn=
2
anan+1
,求数列{bn}的前n项的和.
答案

∵1+2+…+n=

1
2
n(n+1)

an=

1
n+1
+
2
n+1
+…+
n
n+1
=
n
2

bn=

2
n
2
n+1
2
=8(
1
n
-
1
n+1
)

∴数列{bn}的前n项和Sn=8[(1-

1
2
)+(
1
2
-
1
3
)+(
1
3
-
1
4
)+…+(
1
n
-
1
n+1
)]=8(1-
1
n+1
)
=
8n
n+1

完形填空
完形填空。
     Someone says, "Time is money." But I think time is     1     important than money. Why? Because
when money is spent, we can get it back. However, when time is     2    , it'll never     3    . That is    4      
we mustn't waste (浪费) time.
     It goes without saying that the     5     is usually limited (有限的). Even a second is very important. We
should make full use of our time to do     6    useful.
     But it is a pity that there are a lot of people who     7     know the importance of time. They spend their
limited time smoking, drinking and playing.
     They do not know that wasting time means wasting part of their own      8    .
In a word, we should save time. We shouldn't     9     today's work for tomorrow.
     Remember we have no time to    10   .
( )1. A. much
( )2. A. cost  
( )3. A. return
( )4. A. what
( )5. A. money
( )6. A. nothing
( )7. A. do not  
( )8. A. time  
( )9. A. stop  
( )10. A. lose
B. less
B. bought    
B. carry    
B. that    
B. time    
B. something
B. does not  
B. food      
B. leave    
B. save      
C. much less
C. gone        
C. take        
C. because    
C. day        
C. anything    
C. did not    
C. money      
C. let        
C. spend      
D. even more
D. finished  
D. bring    
D. why      
D. food      
D. thing    
D. do        
D. life      
D. give      
D. take      
问答题 简答题