题目

如图，在三棱柱中，侧面为菱形， 且是的中点．(1)求证：∥平面；(2)求证：平面平面. 答案：【答案】（1）证明见解析；（2）证明见解析。【解析】（1）连结C1A，设AC1∩A1C=E，连结DE．由三角形中位线定理得到DE∥BC1．由此能证明BC1∥平面A1DC； （2）由已知条件得△A1AB为正三角形，从而得到 ，AB⊥CD，进而得到AB⊥平面A1DC，由此能证明平面A1DC⊥平面ABC．（1）证明：连结，设，连结．∵三棱柱 Here are some tips on listening. We hope it can help you in one way or another. ●Relax yourself. Before you start to listen to something, You need to relax. 1                                           This will help you with your listening. ●Listen carefully to the first sentence. 2                                          For example, if eyou hear“Many shops in many parts of China have decided to take away all kinds of Japanese goods off their shelves, but some people say it’s not a good idea. Our reporter says…”,you know you will hear a piece of news, not a children’s story or a science report. ●Think when you’re listening. 3                                           For example, you can think of the following questions: What happened? When , where and how? What was the result and what does the speaker want to tell us? In this way, you may undersrand the passage better. ●Listen for important facts. 4                                             For example, if the passage is a science report, you should try to remember its findings , and how the scientists got their results.  ●Don’t think about one or two words for too long. You may hear some words that you don’t know. But don’t spend too much time on them. 5                                          But you can remember some words or sentences that you think are important, and you can use them when you’re answering the questions.   A. The first sentence tells a lot about the whole passage. B. It’s important for you to remember some important facts. C. Don’t get nervous or excited. D. Very often, you’ll find out what they mean later when you go on with    the listening. E. When you’re listening, try to do some thinking.