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设向量组α1，α2，…αt是齐次方程组Ax＝0的一个基础解系，向量β不是方程组Ax＝0的解，即Aβ≠0．试证明：向量组β，β＋α1，β＋α2，…，β＋α1，线性无关．
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Cell Phones: Hang Up or Keep Talking？
Millions of people are using cell phones today. In many places it is actually considered unusual not to use one. In many countries, cell phones are very popular with young people. They find that the phones are more than a means of communication--having a mobile phone shows that they are cool and connected.
The explosions around the world in mobile phone use make some health professionals worded.
Some doctors are concerned that in the future many people may suffer health problems from the use of mobile phones. In England, there has been a serious debate about this issue. Mobile phone companies are worded about the negative publicity of such ideas. They say that there is no proof that mobile phones are bad for your health.
On the other hand, why do some medical studies show changes in the brain cells of some who use mobile phones？ Signs of change in the tissues of the brain and head can be deteeted with modern scanning (扫描) equipment. In one case, a traveling salesman had to retire at a young age because of serious memory loss. He couldn&39;t remember even simple tasks. He would often forget the name of his own son. This man used to talk on his mobile phone for about six hours a day, every day of his working week, for a couple of years. His family doctor blamed his mobile phone use, but his employer&39;s doctor didn&39;t agree.
What is it that makes mobile phones potentially harmful？ The answer is radiation. High-tech machines can detect very small amounts of radiation from mobile phones. Mobile phone companies agree that there is some radiation, but they say the amount is too small to worry about.
As the discussion about their safety continues, it appears that it&39;s best to use mobile phones less often. Use your regular phone if you want to talk for a long time. Use your mobile phone only when you really need it. Mobile phones can be very useful and convenient, especially in emergencies. In the future, mobile phones may have a warning label that says they are bad for your health. So for now, it&39;s wise not to use your mobile phone too often.
People buy cell phones for the following reasons EXCEPT that__________. 查看材料
A） 从9:10 开始的所有事务 B） 从9:10 开始的已确认的事务
C） 从9:10 开始的已改变的页 D） 从9:10 开始的已改变的区
《铁路技术管理规程》中规定，当曲线半径R＜300m时，曲线轨距加宽值为15mm。（)
Centers of the Great European Cities
The centers of the great cities of Europe are meeting places by tradition. People gather there to drink coffee and chat late into the night. A mixture of locals and tourists make for an exciting,metropolitan atmosphere.
Squares, plazas (广场) and arcades (拱廊) form. the heart of Europe&39;s cities.
Venice in Italy has the Piazza San Marco-- a beautiful square surrounded by shops, churches,restaurants and cafes. In Barcelona, Spain, La Bosqueria is a lively market with hundreds of stalls selling all kinds of goods. London&39;s Covent Garden is filled with fruit and vegetable stalls by day and musicians, acrobats (杂技演员) and artists by night. The government buildings at.the center of ma.ny cities often are architecturally impressive. In London, they serve as a beautiful backdrop (背景) to the coffee tables that line the streets and the banks of the Thames.
These vibrant (有活力的 ) hearts are the product of centuries of evolution, social historian Joel Garreau told US News and Worm Report recently. "The reason people think Venice is so great today is you don&39;t see all the mistakes." said Garreau. "Those have all been removed." Most European cities were laid out before the invention of the car, so bars, restaurants and cafes were near to people&39;s homes. Today, the focus of many Europeans&39; life has moved away from the centers They live in the suburbs and outskirts, driving to supermarkets to get their supplies. But on a continent where people treasure convention, there are still those who hold onto traditional ways,living and shopping locally. These people, together with tourists, provide the city centers with their reason for existence.
Coffee culture plays a part in keeping these city centers flourishing. This is particularly true of Paris whose citizens are famous enthusiastic conversationalists. This skill is developed over many hours spent chatting over espressos (浓咖啡) and cigarettes.
Religion also plays a role in developing sociable atmosphere. People in Roman Catholic countries used to visit the church on an almost daily basis. Entire communities would gather in the same building and then move out to the markets, cafes and bars in the surrounding streets. An enormous example of this relationship between church and society is the Duomo. The huge marble cathedral in Florence, Italy is surrounded by bakeries and coffee shops, and caters not only to the tourist crowds, but also the local community.
It can be inferred from the first paragraph that each big city in Europe 查看材料
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