单选题对于微分方程y"+y=sinx，利用待定系数法求其特解y * 时，下面特解设法正确的是______

单选题对于微分方程y"+3y"+2y=e -x ，利用待定系数法求其特解y * 时，下面特解设法正确的是______

单选题When Rupert Murdoch sees beams of light in the American advertising market, it is not necessarily time to reach for the sunglasses. Last October, when the impact of September 11th was only beginning to tell, the boss of NASCAR, a media group, had already identified " strong rays of sunshine". With ad sales still languishing, Mr. Murdoch declared last month that " there are some hints of a modest upswing in tile US advertising market". His early optimism turned out to be misplaced. Now, however, other industry observers are beginning to agree with him. Advertising usually exaggerates the economic cycle, falling sharply and early in a downturn, and rebounding strongly once the economy has begun to recover. This is because most managers prefer to trim their ad budgets rather than their payrolls, and restore such spending only once they feel sure that things are looking up. Last year, America's ad market shrank by 9. 8% , according to CMIR, a research firm. Although ad spending has not yet recovered across all media, some analysts now expect overall ad spending to start to grow in the third quarter. The signs of improvement are patchy, however. Ad spending on radio and television seems to be inching up—advertising on American National Radio was up 2% in January on the same period last year, according to Aegis—while spending on magazines and newspapers is still weak. Even within any one market, there are huge differences; just pick up a copy of one of the now-slimline high-teeh magazines that once bulged with ads, and compare it with the hefty celebrity or women's titles. Advertisers in some categories, such as the travel industry, are still reluctant to buy space or airtime, while others, such as the car and movie businesses, have been bolder. The winter Olympics, held last month in Salt Lake City, has also distorted the spending on broadcast advertising in the first quarter. Nonetheless, there is an underlying pattern. One measure is the booking of ad spots for national brands on local television. By early March, according to Mr. Westerfield's analysis, such bookings were growing fast across eight out of the top ten advertising sectors, led by the financial and motor industries. UBS Warburg now expects the " upfront" market, which starts in May when advertisers book advance ad spots on the TV networks for the new season in September, to be up 4% on last year. On some estimates, even online advertising could pick up by the end of the year.

单选题What's your earliest childhood memory? Can you remember learning to walk? Or talk? The first time you heard thunder or watched a television program? Adults seldom (1) events much earlier than the year or so before entering school, (2) children younger than three or four (3) retain any specific, personal experiences. A variety of explanations have been (4) by psychologists for this "childhood amnesia". One argues that the hippo-campus; the region of the brain which is (5) for forming memories, does not mature until about the age of two. But the most popular theory (6) that, since adults don't think like children, they cannot (7) childhood memories. Adults think in words, and their life memories are like stories or (8) one event follows (9) as in a novel or film. But when they search through their mental (10) for early childhood memories to add to this verbal life story, they don't find any that fit the (11) . It's like trying to find a Chinese word in an English dictionary. Now psychologist Annette Simms of the New York State University offers a new (12) for childhood amnesia. She argues that there simply aren't any early childhood memories to (13) . According to Dr. Simms, children need to learn to use someone else's spoken description of their personal (14) in order to turn their own short-term, quickly forgotten (15) of them into long-term memories. In other (16) , children have to talk about their experiences and hear others talk about (17) --Mother talking about the afternoon (18) looking for seashells at the beach or Dad asking them about their day at Ocean Park. Without this (19) reinforcement, says Dr. Simms, children cannot form (20) memories of their personal experiences.Notes: childhood amnesia 儿童失忆症。

单选题Sadness isn't manly—this Eric Weaver knew. When depression engulfed this New York police sergeant, it took a different guise: a near-constant state of anger. "One minute I'd be okay and the next minute I'd be screaming at my kids and punching the wall," he recalls. "My kids would ask, 'What's wrong with Daddy? Why's he so mad all the time?'" For years, Weaver didn't know what was wrong. Weaver's confusion about what tortured him was not unusual. Roughly a third of the 18 million or more Americans who suffer depression each year are men. Yet all too often, experts say, men fail to recognize the symptoms and get the treatment they need. For years, experts suspected that gender makes a big difference in depression. Studies from New York to New Zealand have repeatedly found the same startling statistic: About twice as many women as men suffer from depression. That finding was considered one of the bedrock facts of modern mental health. Yet it has recently come under attack from critics who are concerned about underreporting of male depression. William Pollack, Director of the Center for Men at McLean Hospital, is leading the charge against the well-entrenched depression gender gap. He argues that men's rate of depression may be nearly equal to women's. Just look at suicide rates, he says: Male suicides outnumber females four to one. That ratio "is way too high to say that men's depression numbers are so low," he notes. Pollack and others contend that male depression goes unrecognized because, unlike the female version, it often doesn't fit the textbook signs—at least in the early stages. Clinical depression at later stages looks much the same in both sexes. But in the prelude to a breakdown, that deepening despair is often expressed in very different ways. Instead of being weepy, men are more apt to be irritable and angry—moods that aren't included in the classic diagnostic tests. "Their sadness and helplessness are hidden behind a mask of anger," says Pollack. "Men tend to act out" to avoid dealing with uncomfortable feelings, adds Fredric Rabinowitz, a psychologist who works primarily with men. If they feel bad, they're apt to get into fights on the job or at home, withdraw from family and friends, become obsessed with work or hobbies. Most significantly, men often turn to drinking or drugs. Men have two to four times the rate of substance abuse problems as women, and Pollack contends that if this was recognized as a sign of depression, the gender gap would substantially narrow.

单选题微分方程y"=y"的通解是______

单选题As one works with color in a practical, or experimental way, one is impressed by two apparently unrelated facts. Color as seen is a mobile, changeable thing (1) to a large extent on the relationship of the color (2) other colors (3) simultaneously. It is not (4) in its relation to the direct stimulus which (5) it. On the other hand, the properties of surfaces that give (6) to color do not seem to change greatly under a wide variety of illumination color, usually (but not always) looking much the same in artificial light as in daylight. Both of these effects seem to be (7) in large part to the mechanism of color (8) . When the eye is (9) to a colored area, there is an immediate readjustment of the (10) of the eye to color in and around the area (11) . This readjustment does not promptly affect the color seen but usually does affect the next area to which the (12) is shifted. The longer the time of viewing, the higher the (13) , and the larger the area, the greater the effect will be (14) its persistence in the (15) viewing situation. As indicated by the work of Wright and Schouten, it appears that, at (16) for a first approximation, full adaptation takes place over (17) time if the adapting source is moderately bright and the eye has been in (18) darkness just previously. Also, (19) of the persistence of the effect if the eye is shifted around from one object to another, all of which are at similar brightness or have similar colors, the adaptation will tend to become (20) over the whole eye.

单选题Time spent in a bookshop can be most enjoyable, whether you are a book-lover or merely there to buy a book as a present. You may even have entered the shop just to find shelter from a sudden shower. But the desire to pick up a book with an attractive dust-jacket is irresistible. You soon become absorbed in some book or other, and usually it is only much later that you realize that you have spent far too much time there. This opportunity to escape the realities of everyday life is, I think, the main attraction of a bookshop. There are not many places where it is possible to do this. A music shop is very much like a bookshop. You can wander round such places to your heart's content. If it is a good shop, no assistant will approach you with the inevitable greeting: "Can I help you, sir?" You needn't buy anything you don't want. In a bookshop an assistant should remain in the background until you have finished browsing. Then, and only then, are his services necessary. You have to be careful not to be attracted by the variety of books in a bookshop. It is very easy to enter the shop looking for a book on, say, ancient coins and to come out carrying a copy of the latest best-selling novel and perhaps a book about brass-rubbing -- something which had only vaguely interested you up till then. This volume on the subject, however, happened to be so well illustrated and the part of the text you read proved so interesting that you just had to buy it. This sort of thing can be very dangerous. Booksellers must be both long suffering and indulgent. There is a story which wei1 illustrates this. A medical student had to read a textbook which was far too expensive for him to buy. He couldn't obtain it from the library and the only copy he could find was in his bookshop. Every afternoon, therefore, he would go along to the shop and read a little of the book at a time. One day, however, he was dismayed to find the book missing from its usual place and about to leave when he noticed the owner of the shop beckoning to him. Expecting to be reproached, he went toward him. To his surprise, the owner pointed to the book, which was tucked away in a corner. "I put it there in case anyone was tempted to buy it," he said, and left the delighted student to continue his reading.

单选题Every second, 1 hectare of the world's rainforest is destroyed. That's equivalent to two football fields. An area the size of New York City is lost every day. In a year, that adds up to 31 million hectares--more than the land area of Poland. This alarming rate of destruction has serious consequences for the environment; scientists estimate, for example, that 137 species of plant, insect or animal become extinct every day due to logging. In British Columbia, where, since 1990, thirteen rainforest valleys have been clearcut, 142 species of salmon have already become extinct, and the habitats of grizzly bears, wolves and many other creatures are threatened. Logging, however, provides jobs, profits, taxes for the government and cheap products of all kinds for consumers, so the government is reluctant to restrict or control it. Much of Canada's forestry production goes towards making pulp and paper. According to the Canadian Pulp and Paper Association, Canada supplies 34% of the world's wood pulp and 49% of its newsprint paper. If these paper products could be produced in some other way, Canadian forests could be preserved. Recently, a possible alternative way of producing paper has been suggested by agriculturalists and environmentalists: a plant called hemp. Hemp has been cultivated by many cultures for thousands of years. It produces fiber which can be made into paper, fuel, oils, textiles, food, and rope. For many centuries, it was essential to the economies of many countries because it was used to make the ropes and cables used on sailing ships; colonial expansion and the establishment of a world wide trading network would not have been possible without hemp. Nowadays, ships' cables are usually made from wire or synthetic fibres, but scientists are now suggesting that the cultivation of hemp should be revived for the production of paper and pulp. According to its proponents, four times as much paper can be produced from land using hemp rather than trees, and many environmentalists believe that the large-scale cultivation of hemp could reduce the pressure on Canada's forests. However, there is a problem : hemp is illegal in many countries of the world. This plant, so useful for fiber, rope, oil, fuel and textiles, is a species of cannabis, related to the plant from which marijuana is produced. In the late 1930s, a movement to ban the drug marijuana began to gather force, resulting in the eventual banning of the cultivation not only of the plant used to produce the drug, but also of the commercial fiber-producing hemp plant. Although both George Washington and Thomas Jefferson grew hemp in large quantities on their own land, any American growing the plant today would soon find himself in prison--despite the fact that marijuana cannot be produced from the hemp plant, since it contains almost no THC (the active ingredient in the drug). In recent years, two major movements for legalization have been gathering strength. One group of activists believes that ALL cannabis should be legal--both the hemp plant and the marijuana plant--and that the use of the drug marijuana should not be an offense. They argue that marijuana is not dangerous or addictive, and that it is used by large numbers of people who are not criminals but productive members of society. They also point out that marijuana is less toxic than alcohol or tobacco. The other legalization movement is concerned only with the hemp plant used to produce fiber; this group wants to make it legal to cultivate the plant and sell the fiber for paper and pulp production. This second group has had a major triumph recently: in 1997, Canada legalized the farming of hemp for fiber. For the first time since 1938, hundreds of farmers are planting this crop, and soon we can expect to see pulp and paper produced from this new source.

单选题For more than a decade, the prevailing view of innovation has been that little guys had the edge. Innovation bubbled up from the bottom, from upstarts and insurgents. Big companies didn't innovate, and government got in the way. In the dominant innovation narrative, venture-backed start-up companies were cast as the nimble winners and large corporations as the sluggish losers. There was a rich vein of business-school research supporting the notion that innovation comes most naturally from small-scale outsiders. That was the headline point that a generation of business people, venture investors and policy makers took away from Clayton M. Christensen's 1997 classic, The Innovator's Dilemma, which examined the process of disruptive change. But a shift in thinking is under way, driven by altered circumstances. In the United States and abroad, the biggest economic and social challenges—and potential business opportunities—are problems in multifaceted fields like the environment, energy and health care that rely on complex systems. Solutions won't come from the next new gadget or clever software, though such innovations will help. Instead, they must plug into a larger network of change shaped by economics, regulation and policy. Progress, experts say, will depend on people in a wide range of disciplines, and collaboration across the public and private sectors. "These days, more than ever, size matters in the innovation game," said John Kao, a former professor at the Harvard business school and an innovation consultant to governments and corporations. In its economic recovery package, the Obama administration is financing programs to generate innovation with technology in health care and energy. The government will spend billions to accelerate the adoption of electronic patient records to help improve care and curb costs, and billions more to spur the installation of so-called smart grids that use sensors and computerized meters to reduce electricity consumption. In other developed nations, where energy costs are higher than in the United States, government and corporate projects to cut fuel use and reduce carbon emissions are further along. But the Obama administration is pushing environmental and energy conservation policy more in the direction of Europe and Japan. The change will bolster demand for more efficient and more environmentally friendly systems for managing commuter traffic, food distribution, electric grids and waterways. These systems are animated by inexpensive sensors and ever-increasing computing power but also require the skills to analyze, model and optimize complex networks, factoring in things as diverse as weather patterns and human behavior. Big companies like General Electric and IBM that employ scientists in many disciplines typically have the skills and scale to tackle such projects.

单选题下列方程为一阶线性微分方程的是______ A. (y')2+2y=x B. y'+2y2=x C. y'+y=x D. y"+y'=x

单选题在下列微分方程中，以y=C1ex+C2cos2x+C3sin2x(C1，C2，C3为任意常数)为通解的是 (A) y'"+y"-4y'-4y=0． (B) y'"+y"+4y'+4y=0． (C) y'"-y"-4y'+4y=0． (D) y'"-y"+4y'-4y=0．

单选题微分方程y"+2y'+y=(x+1)e-x+2x+1有一个特解y*形式为

单选题下列级数中属于条件收敛的是______A．B．C．D．

单选题下列四个级数中发散的是______A．．B．．C．．D．．

单选题已知二阶线性常系数齐次微分方程的通解是y=C1e-2x+C2e3x，则此方程为______ A.y"-y'+6y=0 B.y"-y'-6y=0 C.y"+y'-6y=0 D.y"+y'+6y=0

单选题具有特解y1=e-x，y2=2xe-x，y3=3ex的三阶常系数齐次线性微分方程是A．．B．．C．．D．．

单选题微分方程y"+4y'-5y=0的通解是______ A.y=C1ex+C2e5x B.y=C1e-x+C2e5x C.y=C1ex+C2e-5x D.y=C1e-x+C2e-5x

单选题以下可以看作某个二阶微分方程的通解的函数是 (A) y=C1x2+C2x+C3． (B) x2+y2=C． (C) y=ln(C1x)+ln(C1sinx)． (D) y=C1sin2x+C2cos2x．

单选题已知微分方程y"-4y'+4y=0，函数C1C2xe2x(C1，C2为任意常数)为______ A．方程的通解． B．方程的特解． C．非方程的解． D．是解，但不是通解也不是特解．