在数列{an}中，an=
|
∵1+2+…+n=
n(n+1)1 2
∴an=
+1 n+1
+…+2 n+1
=n n+1 n 2
∴bn=
=8(2
•n 2 n+1 2
-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
)]=8(1-1 n+1
)=1 n+1 8n n+1
| 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 . | ||||
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