题目

如图，已知椭圆的中心在原点，焦点在x轴上，长轴长是短轴长的2倍且经过点M（2，1），平行于OM的直线l在y轴上的截距为m（m≠0），l交椭圆于A、B两个不同点。（1）求椭圆的方程；（2）求m的取值范围；（3）求证直线MA、MB与x轴始终围成一个直角三角形. 答案：解：（1）设椭圆方程为 则∴椭圆方程为 （2）∵直线l平行于OM，且在y轴上的截距为m又KOM=由 ∵直线l与椭圆交于A、B两个不同点，（3）设直线MA、MB的斜率分别为k1，k2，只需证明k1+k2=0即可设 则由而故直线MA、MB与x轴始终围成一个直角三角形.Stop Spam!When I first got an e-mail account ten years ago, I received communications only from family, friends, and colleagues. Now it seems that every time I check my e-mail, I have an endless series of advertisements and other correspondence that do not interest me at all. If we want e-mail to continue to be useful, we need specific laws that make spamming (发送垃圾邮件) a crime.If lawmakers do not do something soon to prohibit spam, the problem will certainly get much worse. Computer programs allow spammers to send hundreds of millions of e-mails almost instantly. As more and more advertisers turn to spam to sell their products, individual (个人的) e-mail boxes are often flooded with spam e-mails. Would people continue to use e-mail if they had to deal with an annoying amount of spam each time?This problem is troubling for individuals and companies as well. Many spam e-mails contain computer viruses that can shut down the entire network of a company. Companies rely on e-mail for their employees to communicate with each other. Spam frequently causes failures in their local communications networks, and their employees are thus unable to communicate effectively. Such a situation results in a loss of productivity and requires companies to repeatedly repair their networks. These computer problems raise production costs of companies, which are, in the end, passes on to the consumer.For these reasons, I believe that lawmakers need to legislate (立法) against spam. Spammers should be fined, and perhaps sent to prison if they continue to disturb people. E-mail is a tool which helps people all over the world to communicate conveniently, but spam is destroying this convenience.1.What does the underlined word “correspondence” in the Paragraph 1 probably mean? A. messages B. ideas C. connections D. programs2.According to the text, what is the major cause of the flooding spam? A. Companies rely on e-mail for communications.B. More people in the world communicate by e-mails.C. Many computer viruses contain spam e-mail.D. More advertisers begin to promote sales through spam.3.According to Paragraph 3, who is the final victim of spam? A. The business B. The advertiser. C. The employee D. The consumer. 4.What is the purpose of the text?A. To inform.B. To educate. C. To persuade. D. To instruct.