已选分类
理学数学
单选题
单选题设n阶矩阵A与B相似，且A+3E不可逆，则B的一个特征值为______。A．B．C．3D．-3
单选题m为偶数(1)设n为整数，m=n(n+1)．(2)在1，2，3，…，1988这1988个自然数中相邻两个数之间任意添加一个加号或减号，设这样组成的运算式的结果是m．
单选题|5-3x|-|3x-2|=3的解是空集。(1)(2)
单选题设线性方程组AX=kβ1+β2有解，其中则k为( )
单选题如图所示，△ABC中，∠ABC＝90°，AB为圆的半径，AB＝20，若面积Ⅰ比面积Ⅱ大7，则AABC的面积等于()．
单选题
单选题已知向量是矩阵的逆矩阵A1的特征向量，则是的值为()．(A)2或-1(B)-2或1
单选题已知a是质数，x，y均为整数，则方程的解的个数是______个．
单选题
单选题已知方程4x
2
+2(m-1)x+(2m+3)=0(m∈R)有两个负根，求m的取值范围______
单选题 There is no question that the old style of air
pollution could kill people. In one week following the infamous "peasouper" fog
in December 1952, 4, 700 people died in London. Most of these people were
elderly and already had heart or lung diseases. A series of these killer fogs
eventually led to the British Parliament passing the Clean Air Act which
restricted the burning of coal. Fortunately the effect of smog
on the lungs is not so dramatic. Scientists have now conducted a number of
laboratory experiments in which volunteers are exposed to ozone inside a steel
chamber for a few hours. Even at quite low concentrations there is a reversible
fall in lung function, an increase in the irritability of the lungs and evidence
of airway inflammation (发炎). Although irritable and inflamed lungs are
particularly seen in people with asthma (哮喘) and other lung diseases, these
effects of ozone also occur in healthy subjects. Similar changes are also seen
after exposure to nitrogen dioxide, although there is some disagreement about
the concentration at which they occur. Other studies have found
that people living in areas with high levels of pollution have more symptoms and
worse lung function than those living in areas with clean air. Groups of
children attending school camps show falls in lung function even at quite low
concentrations of ozone. There is also a relationship between ozone levels and
hospital admissions for asthma, both in North America and Australia. It is
suspected that long-term exposure to smog may result in chronic bronchitis
(支气管炎) and emphysema (肺气肿), but this has yet to be proven.
Recently an association has been found between the levels of particles in the
air and death rates in North American cities. The reason for this association is
not understood and as yet there is no evidence this occurs in Australia.
However, we do know that hazy days are associated with more asthma attacks in
children.
单选题有5本不同的书，其中语文书2本、数学书2本、物理书1本。若将其随机地摆放到书架的同一层上，则同一科目的书都不相邻的概率是______
单选题设函数则f(x)在x=0处______．A．极限不存在B．极限存在但不连续C．连续但不可导D．可导
单选题
单选题在电炉上安装了4个温控器，其显示温度的误差是随机的．在使用过程中，只要有两个温控器显示的温度不低于临界温度t
0
，电炉就断电，以E表示事件“电炉断电”，而T
(1)
≤T
(2)
≤T
(3)
≤T
(4)
为4个温控器显示的按递增顺序排列的温度值，则事件E等于______．
A．{T
(1)
≥t
0
} (13){T
(2)
≥t
0
} C．{T
(3)
≥t
0
} D．{T
(4)
≥t
0
}
单选题
单选题
单选题设函数p(x)，q(x)，f(x)连续，而y1，y2，y3都是方程y"+p(x)y'+q(x)y=f(x)的解，则该方程必定还有解
单选题