单选题设f(x)=3x3+x2|x|，则使f(n)(0)存在的最高阶数n为______．

单选题不定积分等于______A．B．C．-cotx+sinx+CD．cotx+sinx+C

单选题甲、乙两人在环形跑道上跑步，他们同时从起点出发，当方向相反时每隔48秒相遇一次，当方向相同时每隔10分钟相遇一次．若甲每分钟比乙快40米，则甲、乙两人的跑步速度分别是______米/分钟．

单选题具有特解y1=e-x，y2=2xe-x，y3=3ex的3阶常系数齐次微分方程是( )． A．y-y-y+y=0 B．y+y-y-y=0 C．y-6y+11y-6y=0 D．y-2y-y+2y=0

单选题设A，B为n阶矩阵，满足等式AB=0，则必有______

单选题在正三棱柱ABC—A1B1C1中，若AB=BB1，则AB1与C1B所成的角的大小为()。A．60°B．90°C．105°D．75°

单选题设函数f i (x)(i=1，2)具有二阶连续导数，且f" i (x 0 )＜0(i=1，2)，若两条曲线y=f i (x)(i=1，2)在点(x 0 ，y 0 )处具有公切线y=g(x)，且在该点处曲线y=f 1 (x)的曲率大于曲线y=f 2 (x)的曲率，则在x 0 的某个邻域内有______

单选题假设区域D由曲线y=px3(x＞0，p＞0)及其过点(1，p)的切线与x轴围成，设此区域的形心为则的值为______A．B．C．D．

单选题10封不同的信，投到3个相同的邮筒中，若一个邮筒里投2封信，另外两个邮筒各投4封信，不同的投法有()种．

单选题方程(a-1)x—y+2a+1=0(a∈R)所表示的直线( )．

单选题Who is poor in America? This is a hard question to answer. Despite poverty's messiness, we've measured progress against it by a single statistic: the federal poverty line. In 2008, the poverty threshold was $ 21,834 for a four-member family with two children under 18. By 1his measure, we haven't made much progress. Except for recessions, when the poverty rate can rise to 15 percent, it's stayed in a narrow range for decades. In 2007—the peak of the last business cycle—the poverty rate was 12.5 percent; one out of eight Americans was "poor. " In 1969, another business-cycle peak, the poverty rate was 12.1 percent. But the apparent lack of progress is misleading for two reasons. First, it ignores immigration. Many immigrants are poor and low skilled. They add to the poor. From 1989 to 2007, about three quarters of the increase in the poverty population occurred among Hispanics—mostly immigrants, their children, and grandchildren. The poverty rate for blacks fell during this period, though it was still much too high (24.5 percent in 2007). Poverty "experts" don't dwell on immigration, because it implies that more restrictive policies might reduce U.S. poverty. Second, the poor's material well-being has improved. The official poverty measure obscures this by counting only pretax cash income and ignoring other sources of support. These include the earned-income tax credit (a rebate to low-income workers), food stamps, health insurance (Medicaid), and housing subsidies. Although many poor live hand to mouth, they've participated in rising living standards. In 2005, 91 percent had microwaves, 79 percent air-conditioning, and 48 percent cell phones. The existing poverty line could be improved by adding some income sources and subtracting some expenses (example: child care). Unfortunately, the administration's proposal for a "supplemental poverty measure" in 2011—to complement, not replace, the existing poverty line—goes beyond that. The new poverty number would compound public confusion. It also raises questions about whether the statistic is tailored to favor a political agenda. The "supplemental measure" ties the poverty threshold to what the poorest third of Americans spend on food, housing, clothing, and utilities. The actual threshold not yet calculated—will probably be higher than today's poverty line. Moreover, this definition has strange consequences. Suppose that all Americans doubled their income tomorrow, and suppose that their spending on food, clothing, housing, and utilities also doubled. That would seem to signify less poverty—but not by the new poverty measure. It wouldn't decline, because the poverty threshold would go up as spending went up. Many Americans would find this weird., people get richer, but "poverty" stays stuck. What produces this outcome is a different view of poverty. The present concept is an absolute one: the poverty threshold reflects the amount estimated to meet basic needs. By contrast, the new measure embraces a relative notion of poverty: people are automatically poor if they're a given distance from the top, even if their incomes are increasing.

单选题将方程去分母并化简后得到的方程是______。A．x2-2x-3=0B．x2-2x-5=0C．x2-3=0D．x2-5=0

单选题 求由平面与所构成的二面角的角平分面的方程, 在此二面角内有点. A、 B、 C、 D、

单选题设，则f(x)在x=0处()

单选题函数f(x)=x 3 -3x+k只有一个零点，则k的范围为______．

单选题设向量组1=(1,2,3)，2=(0,l,2)，3=(0,0,l)，=(1,3,6)，则

单选题设，则下列命题正确的是A．若条件收敛，则与都收敛．B．若绝对收敛，则与都收敛．C．若条件收敛，则与的敛散性都不定．D．若绝对收敛，则与的敛散性都不定．

单选题设A为m×n矩阵，m≥n，r

单选题已知A是3阶矩阵且则

单选题若在x=0处连续，则a的值是A．0．B．1．C．2．D．