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设有抛物线L：y=a-bx2，试确定常数a，b的值，使得 (Ⅰ)L与直线y=x+1相切； (Ⅱ)L与x轴所围图形绕y轴旋转所得旋转体的体积最大。
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设随机变量X服从正态分布N(μ，σ2)，则随σ的增大，概率P(|X|＜σ)______。
Text 1 Every Saturday morning, at 9 am, more than 50,000 runners set off to run 5km around their local park. The Parkrun phenomenon began with a dozen friends and has inspired 400 events in the UK and more abroad. Events are free, staffed by thousands of volunteers. Runners range from four years old to grandparents; their times range from Andrew Baddeley’s world record 13 minutes 48 seconds up to an hour. Parkrun is succeeding where London’s Olympic "legacy" is failing. Ten years ago on Monday, it was announced that the Games of the 30th Olympiad would be in London. Planning documents pledged that the great legacy of the Games would be to level a nation of sport lovers away from their couches. The population would be fitter, healthier and produce more winners. It has not happened. The number of adults doing weekly sport did rise, by nearly 2 million in the run-up to 2012-but the general population was growing faster. Worse, the numbers are now falling at an accelerating rate. The opposition claims primary school pupils doing at least two hours of sport a week have nearly halved. Obesity has risen among adults and children. Official retrospections continue as to why London 2012 failed to "inspire a generation." The success of Parkrun offers answers. Parkun is not a race but a time trial: Your only competitor is the clock. The ethos welcomes anybody. There is as much joy over a puffed-out first-timer being clapped over the line as there is about top talent shining. The Olympic bidders, by contrast, wanted to get more people doing sports and to produce more elite athletes. The dual aim was mixed up: The stress on success over taking part was intimidating for newcomers. Indeed, there is something a little absurd in the state getting involved in the planning of such a fundamentally "grassroots", concept as community sports associations. If there is a role for government, it should really be getting involved in providing common goods-making sure there is space for playing fields and the money to pave tennis and netball courts, and encouraging the provision of all these activities in schools. But successive governments have presided over selling green spaces, squeezing money from local authorities and declining attention on sport in education. Instead of wordy, worthy strategies, future governments need to do more to provide the conditions for sport to thrive. Or at least not make them worse. The author’s attitude to what UK governments have done for sports is_____.
Text 4 Though often viewed as a problem for western states, the growing frequency of wildfires is a national concern because of its impact on federal tax dollars, says Professor Max Moritz, a specialist in fire ecology and management. In 2015, the US Forest Service for the first time spent more than half of its $5.5 billion annual budget fighting fires-nearly double the percentage it spent on such efforts 20 years ago. In effect, fewer federal funds today are going towards the agency’s other work-such as forest conservation, watershed and cultural resources management, and infrastructure upkeep-that affect the lives of all Americans. Another nationwide concern is whether public funds from other agencies are going into construction in fire-prone districts. As Moritz puts it, how often are federal dollars building homes that are likely to be lost to a wildfire "It’s already a huge problem from a public expenditure perspective for the whole country," he says. We need to take a magnifying glass to that. Like, "Wait a minute, is this OK" "Do we want instead to redirect those funds to concentrate on lower-hazard parts of the landscape" Such a view would require a corresponding shift in the way US society today views fire, researchers say. For one thing, conversations about wildfires need to be more inclusive. Over the past decade, the focus has been on climate change-how the warming of the Earth from greenhouse gases is leading to conditions that worsen fires. While climate is a key element, Moritz says, it shouldn’t come at the expense of the rest of the equation. "The human systems and the landscapes we live on are linked, and the interactions go both ways," he says. Failing to recognize that, he notes, leads to "an overly simplified view of what the solutions might be. Our perception of the problem and of what the solution is becomes very limited." At the same time, people continue to treat fire as an event that needs to be wholly controlled and unleashed only out of necessity, says Professor Balch at the University of Colorado. But acknowledging fire’s inevitable presence in human life is an attitude crucial to developing the laws, policies, and practices that make it as safe as possible, she says. "We’ve disconnected ourselves from living with fire," Balch says. "It is really important to understand and try and tease out what is the human connection with fire today." Professor Balch points out that fire is something man should _____.
Text 3 Today, widespread social pressure to immediately go to college in conjunction with increasingly high expectations in a fast-moving world often causes students to completely overlook the possibility of taking a gap year. After all, if everyone you know is going to college in the fall, it seems silly to stay back a year, doesn’t it And after going to school for 12 years, it doesn’t feel natural to spend a year doing something that isn’t academic. But while this may be true, it’s not a good enough reason to condemn gap years. There’s always a constant fear of falling behind everyone else on the socially perpetuated "race to the finish line," whether that be toward graduate school, medical school or lucrative career. But despite common misconceptions, a gap year does not hinder the success of academic pursuits-in fact, it probably enhances it. Studies from the United States and Australia show that students who take a gap year are generally better prepared for and perform better in college than those who do not. Rather than pulling students back, a gap year pushes them ahead by preparing them for independence, new responsibilities and environmental changes-all things that first-year students often struggle with the most. Gap year experiences can lessen the blow when it comes to adjusting to college and being thrown into a brand new environment, making it easier to focus on academics and activities rather than acclimation blunders. If you’re not convinced of the inherent value in taking a year off to explore interests, then consider its financial impact on future academic choices. According to the National Center for Education Statistics, nearly 80 percent of college students end up changing their majors at least once. This isn’t surprising, considering the basic mandatory high school curriculum leaves students with a poor understanding of themselves listing one major on their college applications, but switching to another after taking college classes. It’s not necessarily a bad thing, but depending on the school, it can be costly to make up credits after switching too late in the game. At Boston College, for example, you would have to complete an extra year were you to switch to the nursing school from another department. Taking a gap year to figure things out initially can help prevent stress and save money later on. The most suitable title for this text would be_____.
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