题目

已知椭圆上一点A到左焦点的距离为，则点Ａ到直线的距离为                                                                        （  ）    A．             B．    C．     D． 答案：解析：设左右焦点为，则，. 椭圆的离心率为.而即为右准线，由定义得，A到直线的距离等于。Dear sirI'm a middle school student. I have something to talk with you. In my school, many students have mobile phones（手机）.They think it's cool to have a mobile phone. Really the phones come in fun colours and let students talk with their parents and friends at any time. But I don't think it's good to send messages（短信）.It takes them a lot of time to do that. Some students always hope to buy the latest mobile phones and this will waste（浪费）their parents' money. And my father tells me that most mobile phones can cause health problems. Could you do something to stop it? Yours Mary【小题1】Students use mobile phones to_________.A．talk with their parents and friends and send messagesB．learn from othersC．study EnglishD．talk with their teachers【小题2】The students want to have the latest mobile phones. And it will_________.A．waste their parents' timeB．waste their parents' moneyC．save their timeD．save their money【小题3】The writer thinks that students_________.A．can't use a mobile phone at homeB．should use mobile phones at any timeC．must have a mobile phone at schoolD．shouldn't use a mobile phone at school