单选题的值______

单选题设二维随机变量(X，y)的联合密度函数为则k为()

单选题已知a，b是方程x 2 -4x+m=0的两个根，b，c是方程x 2 -8x+5m=0的两个根，则m=______．

单选题设，则F"(1)=______

单选题D(X-3Y+1)=（）

单选题Scientists around the world are racing to learn how to rapidly diagnose, treat and stop the spread of a new, deadly disease. SARS — Severe Acute Respiratory Syndrome — was (1) for the first time in February 2003 in Hanoi, (2) since then has infected more than 1,600 people in 15 countries, killing 63. At this (3) , there are more questions than answers surrounding the disease. Symptoms start (4) a fever over 100.4 degrees F, chills, headache or body (5) . Within a week, the patient has a dry cough, which might (6) to shortness of breath. In 10% to 200% of cases, patients require (7) ventilation to breathe. About 3.5% die from the disease. Symptoms (8) begin in two to seven days, but some reports suggest it (9) take as long as 10 days. Scientists are close to (10) a lab test to diagnose SARS. In the meantime, it is diagnosed by its symptoms. There is no evidence (11) antibiotics or anti-viral medicines help, (12) doctors can offer only supportive care. Patients with SARS are kept in isolation to reduce the risk of (13) . Scientists aren’t sure yet, but some researchers think it’s a (14) discovered coronavirus, the family of viruses that cause some common colds. Most cases appear to have been passed (15) droplets expelled when infected patients cough or sneeze. Family members of infected people and medical workers who care for them have been most likely to (16) the illness. But recent developments in Hong Kong suggest that the (17) might spread through air, or that the virus might (18) for two to three hours on doorknobs or other (19) Health experts say it is (20) , though, that sharing an elevator briefly with an infected person would be enough to pass the virus.

单选题设级数其中α，β，γ均为大于1且与n无关的常数，则______

单选题f(x)=x2+x-1，g(x)=a(x+1)2+b(x-1)(x+1)+c(x-1)2，a，b，c为()时，f(x)=g(x)．

单选题函数的最小正周期是()．

单选题设函数f(u)可导，y=f(x 2 )，当自变量x在x=-1处取得增量Δx=-0.1时，相应的函数增量Δy的线性主部为0.1，则f"(1)=

单选题设矩阵，已知矩阵A相似于B，则秩(A-2E与秩(A-E)的和等于()．

单选题设函数，则f(y，x)等于______。A．B．C．D．

单选题When older people can no longer remember names at a cocktail party, they tend to think that their brainpower is declining. But a growing number of studies suggest that this assumption is often wrong. Instead, the research finds, the aging brain is simply taking in more data and trying to sift through a clutter of information, often to its long-term benefit. The studies are analyzed in a new edition of a neurology book, "Progress in Brain Research. " Some brains do deteriorate with age. Alzheimer's disease, for example, strikes 13 percent of Americans 65 and older. But for most aging adults, the authors say, much of what occurs is a gradually widening focus of attention that makes it more difficult to latch onto just one fact, like a name or a telephone number. Although that can be frustrating, it is often useful. "It may be that distractibility is not, in fact, a bad thing," said Shelley H. Carson, a psychology researcher at Harvard whose work was cited in the book. "It may increase the amount of information available to the conscious mind. " For example, in studies where subjects are asked to read passages that are interrupted with unexpected words or phrases, adults 60 and older work much more slowly than college students. Although the students plow through the texts at a consistent speed regardless of what the out-of-place words mean, older people slow down even more when the words are related to the topic at hand. That indicates that they are not just stumbling over the extra information, but are taking it in and processing it. When both groups were later asked questions for which the out-of-place words might be answers, the older adults responded much better than the students. "For the young people, it's as if the distraction never happened," said an author of the review, Lynn Hasher, a professor of psychology at the University of Toronto and a senior scientist at the Rotman Research Institute. "But for older adults, because they've retained all this extra data, they' re now suddenly the better problem solvers. They can transfer the information they've soaked up from one situation to another. " Such tendencies can yield big advantages in the real world, where it is not always clear what information is important, or will become important. A seemingly irrelevant point or suggestion in a memo can take on new meaning if the original plan changes. Or extra details that stole your attention, like others'yawning and fidgeting, may help you assess the speaker's real impact.

单选题设集合M={x||x—2|2}，则M∩N=(　　)

单选题下列函数f(x)中，导函数f"(x)在x=0处不连续的是A．B．C．D．

单选题已知多项式2x4-3x3-ax2+7x+b能被x2+x-2整除，则=()．

单选题当x→0时，下列函数以零为极限的是()。

单选题设，则x=1为f(x)在[-2，2]上的____

单选题设f(x)是奇函数，其中a为不等于1的正数，则F(x)是{{U}}{{/U}}。A.偶函数B.奇函数C.非奇非偶函数D.奇偶性与a有关