题目

已知数列是公差不为0的等差数列，对任意大于2的正整数，记集合的元素个数为，把的各项摆成如图所示的三角形数阵，则数阵中第17行由左向右数第10个数为___________．                                                                   ………… 答案：_293______．              【解析】设，则，由题意，当，时， 取最小值1，当，时，取最大值，易知可取遍，即.数阵中前16行共有个数，所以第17行左数第10个数为We all dream although we remember only a few of our dreams. Some people train themselves to remember their dreams. As soon as they wake up, they record what their dreams were about. Scientists believe that we have different kinds of sleep. One kind is called R.E.M. R.E.W. stands for rapid eye movement. During this kind of sleep our eyes move quickly, even though they are closed. Although we are asleep, there is a lot of brain activities. We probably have dreams at this time. Sometimes people talk in their sleep. In these dreams the dreamer is usually unhappy or worried about something. The worst of the dreams are so frightening that we call them nightmares. In a nightmare we often dream that we are being closed () or that we are trapped in some way. Scientists or others have written books about the meanings of dreams. The most famous of these scientists was Sigmund Freud who wrote a hundred years ago. He suggested different meanings to certain kinds of dreams. It is possible, however, that every dream has a special meaning only for the person who dreams it, and that this dream about something in that person’s life. 1. How can we tell if someone is having an R.E.M. dream?    A. They are talking in their sleep.              B. They are worried about something.    C. Their eyes are moving quickly.               D. heir eyes are moving slowly. 2. What does the word “nightmare” in the 3rd paragraph means in Chinese?    A. 噩梦              B. 睡眠               C. 美梦             D. 失眠 3. What is the main idea of Paragraph Four?    A. Bad Dreams                                   B. The meaning of dreams     C. People and their dreams                  D. R.E.M. and sleep and dreams