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设u=f(x2+y2，xz)，z=z(x，y)由ex+ey=ez确定，其中f二阶连续可偏导，求．
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Thanks to the Internet, an alternative to the traditional unhappy model of whichsupplier-customer interaction is finally becoming possible. In all sorts of markets,41．customers will soon be able to describe exactly what they want, and suppliers will be42．able to deliver what the desired product or service without compromise or delay.43．The innovation that will catalyze into this shift is what I call the choiceboard.44．Choiceboards are interactive, on-line systems that allow individual customers to design for45．their own products by choosing from a menu of attributes, or components, prices, and46．delivery options. The customers’ selections send signals to the supplier’s manufacturing47．system that set it in motion the wheels of procurement, assembly and delivery.48．The role of the customer in this system shifts from a passive recipient to active designer.49．That shift is just the most recent stage in the long-term evolution of the customer’s role of50．in the economy. For most of the twentieth century, the customers were "product takers" and51．"price takers", accepting suppliers’ goods at suppliers prices. Over the past two decades,52．as customers became more sophisticated and being gained greater power over the buying process, they stopped being price takers. 51（）
It can be tempting to make a hasty decision when a killer opportunity comes along or the thought of spending another day on the job seems painful. (67) , Career coach Piotrowski recommends taking baby (68) to execute a new career strategy. "Plan a timeline of one to two years to (69) your career change. Gather information for four to six months, and then get moving on activities that will (70) into your new specialty over the next few months. Remember, you can make the (71) over time. You don’t need to do it all at (72) ." "Spend time looking (73) industry categories and a variety of jobs to get ideas about new career areas that may (74) to you. This can open your eyes to a multitude of (75) you hadn’t considered before." Informational interviews--the best-kept career-change secret, according to Piotrowski--will also help career changers come to a(n) (76) . The key is to seek people already lost in a (77) career and pick their brain with questions such as, " (78) training do I need to do well in this job, what kind of money will I (79) , and what’s a day on the job really like" Finally, people should try a few career experiments to (80) their abilities and build experience to help them move into a new career more (81) ."A career experiment can be one of thousands of activities that (82) you to learn more about a new type of work (83) you commit to choosing it." Career experiments (84) shadowing a specialist, volunteering, (85) field trips, and designing projects to (86) your knowledge and skills.
Questions 1-2Choose the correct answer A, B, C or D. Thieves often target students’ homes because students（）
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