题目

如图，几何体P－ABCD中，底面ABCD为直角梯形，侧面PAD为等边三角形，且CD∥AB，∠DAB＝90°，CD＝DA＝AB＝1，PB＝. (Ⅰ)求证：面PAD⊥面ABCD； (Ⅱ)求二面角A－PB－C的平面角的余弦值． 答案：【解析】(Ⅰ)由于PA＝1，AB＝3，PB＝， 则PB2＝PA2＋AB2，则BA⊥PA， 又∠DAB＝90°，则BA⊥DA，故BA⊥面PAD， 又BA⊂面ABCD，则面PAD⊥面ABCD. (Ⅱ)取O为AD中点，建立如图所示空间直角坐标系O－xyz.取E为PA中点， 则n＝，而显然二面角A－PB－C为锐二面角(直接由CH与DE平行且相等知点H在△PAB的内部)，故所求余弦值为Most people nowadays are persistently seeking for opportunities that will assist them in reaching their goals. Marketing yourself is when there is a demand for you and your services that will help you achieve your goals. Marketing yourself means you are simply “selling” yourself! The common problem in marketing yourself is that most people think that it is nasty to do this.They think that marketing concerns only to a business. Of course, this is absolutely not true. People who want to attain the success they desire must promote themselves. In every attempt you make, just think that marketing yourself is a very useful tool. The better you market yourself, the more opportunities you will have. Marketing yourself does not really mean telling every person how beautiful and wonderful you are. It simply means telling your prospective employer that you exist, that you have the possibility of generating much greater income than your prospective employer makes. Or maybe you have the chance of doing something better. There are actually several different ways of marketing yourself.But there are only two primary approaches that you implement(实施)．The first is you find ways to reach out to others and the second is to construct ways in order for the people to find you and make connection with you. In order for you to reach out and touch the lives of others, you have to be a good person and a good employee.In that way your good reputation will follow naturally. Have a brief and concise(简明) message that you can share with others.When you market yourself, you tell people who you are and what you can do for others.Plan well and practice your message. In constructing ways for the people to notice and make connection with you, you must be open to them. Bear in mind that communication is a great opportunity to make an impact especially when you market yourself. You also communicate by what you are wearing.Know the appropriate dress code and have plenty of time to decide what you will wear. Just remember to make a great first impression! 1．According to the passage, in general people tend to think marketing yourself________. A．is a very useful tool B．will make you have more opportunities C．is nasty and concerns only to a business D．is natural 2．All of the following are true about the marketing yourself EXCEPT________. A．if you can marketing yourself better, you will have more opportunities B．if you want to succeed, you must learn how to market yourself C．marketing yourself simply means making your boss know you can make more money than him D．marketing yourself really means telling others you are beautiful and wonderful 3．In order to market yourself well, you can do these things EXCEPT________. A．praise yourself greatly 　　　 B．wear appropriately C．have a brief and clear message 　 D．be open to others 4．It can be concluded from the passage that________. A．there are only two ways of marketing yourself B．communication plays an important part in marketing yourself C．you needn’t spent plenty of time to decide what you will wear D．marketing yourself simply means telling your employer you must earn much greater than him