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问题 解答题

已知：关于x的一元二次方程x2+kx-4=0．

（1）求证：方程有两个不相等的实数根．

（2）设方程的两根分别为x1、x2，且满足x1+x2=x1•x2，求k的值．

答案

（1）△=b2-4ac=k2-4×（-4）=k2+16，

∵k2≥0，

∴△＞0，

∴方程有两个不相等的实数根；

（2）根据题意可得，

x1+x2=-

b
a
=-k，x1x2=
c
a
=-4，

∵x1+x2=x1•x2，

∴-k=-4，

∴k=4．

判断题

泵的平衡装置造成工作水的泄漏量一般占工作流量的10%。
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单项选择题

Advertisers tend to think big and perhaps this is why they’re always coming in for criticism. Their critics seem to resent them because they have a flair for self-promotion and because they have so much money to threw around. "It’s iniquitous," they say, "that this entirely unproductive industry (if we can call it that) should absorb millions of pounds each year. It only goes to show how much profit the big companies arc making. Why don’t they stop advertising and reduce the price of their goods After all, it’s the consumer who pays."
The poor old consumer. He would have to pay a great deal more if advertising didn’t create mass markets for products. It is just because of the heavy advertising that consumer goods are so cheap. But we get the wrong idea if we think the only purpose of advertising is to sell goods. Another equally important function is to inform. A great deal of the knowledge we have about household goods derives large from the advertisements we read. Advertisements introduce us to new products or remind us of the existence of ones we already know about. Supposing you wanted to buy a washing-machine, it is more than likely you would obtain details regarding performance, price, etc. from an advertisement.
Lots of people pretend that they never read advertisements, but this claim may be seriously doubted. It is hardly possible not to read advertisements these days. And what fun they often are, too] Just think what a railway station or a newspaper would be like without advertisements. Would you enjoy gazing at a blank wall or reading railway by-laws while waiting for a train Would you like to read only closely-printed columns of news in your daily paper A cheerful, witty advertisement makes such a difference to a drab wall or a newspaper full of the daily ration of calamities.
We must not forget, either, that advertising makes a positive contribution to our pockets. Newspapers, commercial radio and television companies could not subsist without this source of revenue. The fact that we pay so little for our daily paper, or can enjoy so many broadcast programs is due entirely to the money spent by advertisers. Just think what a newspaper would cost if we had to pay its full price !
Another thing we shouldn’t forget is the "little ads", which are in virtually every newspaper and magazine. What a tremendously useful service they perform for the community ! Just about anything can be accomplished through these columns. For example, you can find a job, buy or sell a house, announce a birth, marriage or death in what used to be called the "hatch, match and dispatch" columns; but by far the most fascinating section is the personal or "agony" column. No other item in a newspaper provides such entertaining reading or offers such a deep insight into human nature. It’s the best advertisement for advertising there is!

The author mentions the example of a washing-machine to justify______

A. informativeness of ads.
B. credulity of consumers.
C. deception of companies.
D. techniques of advertisers.
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