题目

已知的顶点，的坐标分别为，，若点在抛物线上移动，求的重心的轨迹． 答案：的重心的轨迹为开口向上的抛物线，但不包括点和． 解析:设的重心的坐标为，顶点的坐标为，则． 则重心坐标公式得 代入得 ，化简整理得． 又直线的方程为，即． 由得或 而，，三点不在同一直线上， ，，三点不共线． 即轨迹中应去掉到和． 故的重心的轨迹为开口向上的抛物线，但不包括点和． 第三部分 阅读理解 (共20小题；每小题2分，满分40分) 阅读下列短文，从每题所给的四个选项(A、B、C和D)中，选出最佳选项。 People enjoy taking trips．But what are the reasons they leave home? One reason is for education．People travel because they want to broaden their horizons to learn about other people and other places．They are curious about other cultures．When people are tourists, they get a quick look at different ways of living．Even a short look at another kind of lifestyle is an important lesson．On a trip, a person can learn directly by visiting museums and historic spots．What does a tourist learn who sees the art museums, visits the historical places and other scenic spots in Paris and shops along the River Seine? He gets a vivid picture, a real life of one of the French people．He learns about their attitudes, how they feel about business; beauty and history．What about the tourist who goes to Hong Kong? Does he get the same information that he could get from a book? He might read that Hong Kong is crowded, that there is less than 200 square meters of space for each person．But seeing and feeling the lack of space will impress him much more．He might read that there are nearly 200 vehicles for every kilometer of roadway．But the sight of so many vehicles parked along the roadside would be a much more vivid lesson．The tourist to Hong Kong will never forget the contrasts: the straight vertical lines of tall modern buildings and the moving lines of boats that people live in. 1．Why do people leave home to travel according to passage? A．For education.                           B．For adventure. C．To enjoy themselves.                  D．To look for a different lifestyle. 2．What do we learn from the passage about Paris? A．It has a dense population. B．It has many towering buildings. C．There are 200 vehicles for every kilometer of roadway. D．There are many museums and palaces. 3．What impression will a tourist get of Hong Kong? A．It has many big and beautiful parks. B．It is a city of contrasts. C．It possesses many historical sites. D．It is an important industrial center. 4．What does the passage tell us about traveling? A．It makes our life more interesting. B．It enables us to get first-hand knowledge. C．It helps develop our personalities. D．It brings about changes in our lifestyle.