问题
选择题
已知数列{an}的前n项和Sn=-n2+6n，则当Sn取最大值时，n等于（ ）
A．2
B．3
C．2或3
D．3或4
答案
Sn=-n2+6n=-（n-3）2+9
∴当n=3时，Sn取最大值9
故选B．
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已知数列{an}的前n项和Sn=-n2+6n，则当Sn取最大值时，n等于（ ）
A．2
B．3
C．2或3
D．3或4
Sn=-n2+6n=-（n-3）2+9
∴当n=3时，Sn取最大值9
故选B．
| 完形填空。 | |||
| Bernard Shaw wasn't interested in clothes. When he was young, he was too poor to 1 good clothes. When he was 2 , he did not wear good clothes, either. In early days his clothes were 3 old that the colors in the material faded (褪色) over the years. Later in his life he was glad to dress differently from 4 men. When they went out in evening dresses in the evening, Shaw did not. Even when he went to the theater, he was in an old coat. It was made 5 a kind of material that he liked. One evening he went to see a play in his old coat. At the door of the theater the gatekeeper 6 him from entering it. " 7 is the matter?" he asked angrily. The man pointed to the old coat. "Don't you like it?" Shaw cried, "If you don't like it, I'll 8 . " A few minutes later, he walked towards his place in the theater 9 the coat, but the man followed him, crying loudly, "Stop! You can't go in like that!" Shaw's 10 face turned black, "Do you think I'm going to take off any more?" | |||
| ( )1. A. put on ( )2. A. older ( )3. A. such ( )4. A. other ( )5. A. into ( )6. A. asked ( )7. A. How ( )8. A. take it off ( )9. A. in ( )10. A. angry | B. buy B. stronger B. so B. others B. in B. stood B. What B. take off it B. on B. happy | C. sell C. taller C. very C. another C. by C. stopped C. Which C. put it on C. with C. sad | D. had D. younger D. too D. the other D. of D. protected D. Why D. put on it D. without D. worried |