问题 填空题

一个数列,如果每一项与它的后一项的和都为同一个常数,那么这个数列叫做等和数列,这个常数叫做该数列的公和.已知数列{an}是等和数列,且a1=2,公和为5,那么这个数列的前21项和S21的值为______.

答案

由题意知,an+an+1=5,且a1=2,所以,a1+a2=5,得a2=3,a3=2,a4=3,…a20=3,a21=2,

∴S21=(2+3)+(2+3)+…(2+3)+2=5×10+2=52

故答案为:52

问答题 简答题
完形填空

A lady and her husband stepped off the train in Boston. They walked without an appointment (预约) into the outer ___36___of Harvard’s president. But they were ___37___ by his secretary and kept waiting. For hours, the secretary took no notice of them, ___38___ that the couple would finally become ___39___ and go away. But they didn’t. The secretary finally decided to disturb the president, though ___40___.

A few minutes later, the president walked towards the couple with a ___41___face. The lady told him, “We had a son that ___42___Harvard for one year. He loved Harvard. He was ___43___here. But about a year ago, he was accidentally killed. My hus­band and I would like to ___44___a memorial (纪念物) to him, somewhere on campus.”

The president wasn’t ___45___Instead, he was shocked. “Madam,” he said, “we can’t put up a statue for every person who studied at Harvard and died. If we did, this ___46___would look like a cemetery (墓地).” “Oh, no,” the lady ___47___quickly, “we don’t want to put up a statue. We would like to give a ___48___to Harvard. “The president rolled his eyes and ___49___at the couple and then exclaimed, “A building! Do you have any ___50___how much a building costs? We have spent over $ 7,500,000 on the cam­pus building at Harvard.” For a moment the lady was silent. The president was ___51___, because he could get rid of them now. Then the lady turned to her husband and said quietly. “Is that all it costs to start a ___52___? Why don’t we just start our own?” Her husband nodded. ___53___their offer was turned down. Mr, and Mrs Stanford traveled to California where they founded the University that bears their ___54___, a me­morial to a son that Harvard no longer ___55___ about.

36. A. Lab      B. library       C. hall     D. off ice

37. A. watched       B. stopped      C. followed    D. interviewed

38. A. hoping B. finding       C. realizing     D. imaging

39. A. surprised     B. disappointed C. worried    D. troubled

40. A. hopelessly    B. carefully    C. unexpected D. unwillingly

41. A. pleasant       B. funny  C. cold    D. sad

42. A. attended      B. visited C. studied       D. served

43. A. clever   B. brave  C. proud  D. happy

44. A. set about      B. set up  C. take down  D. take over

45. A. satisfied       B. excited       C. moved       D. ashamed

46. A. house   B. part    C. garden       D. place

47. A. explained     B. expressed   C. refused       D. admitted

48. A. building      B. yard    C. playground D. square

49. A. laughed       B. shouted      C. glanced      D. called

50. A. suggestion   B. idea    C. thought      D. opinion

51. A. bored   B. astonished  C. interested   D. pleased

52. A. department  B. university   C. business     D. club

53. A. Once    B. While C. Since  D. Though

54. A. name    B. character    C. picture       D. sign

55. A. talked   B. knew  C. beard  D. cared