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，不同的投法种数是______A．B．C．D．E．

单选题设A，B都是n阶可逆矩阵，则下列选项中正确的是 A．A+B可逆． B．|A|=|B|． C．A经过行的初等变换可变为B． D．存在可逆矩阵P，使得P-1AP=B．

单选题某外商计划在4个候选城市投资3个不同的项目，且在同一个城市投资的项目不超过2个，则该外商不同的投资方案有( )种．

单选题设f(x)为可导函数，则等于()。

单选题设a-b=-2，则a3+6ab-b3=( )．

单选题要使两个相邻正整数的值可以确定．(1)它们的平方和大于61(1)若将它们都减去1，则所得两数之比小于

单选题设无穷等比数列所有奇数项之和为15，所有偶数项之和为-3，a1为其首项，则a1={{U}}{{/U}}．

单选题若在[0，1]上有f(0)=g(0)=0，f(1)=g(1)=a＞0，且f"(x)＞0，g"(x)＜0，则的大小关系是A．I1≥I2≥I3．B．I3≥I2≥I1．C．I2≥I3≥I1．D．I2≥I1≥I3．

单选题Here in the U. S. a project of moving the government a few hundred miles to the southwest proceeds apace, under the supervision of Senator Robert Byrd of West Virginia. Apart from the usual highways and parks, Byrd has taken a special interest in transplanting pieces of federal agencies from metropolitan Washington to his home state. Strangely, Byrd's little experiment in de-Washingtonization has become the focus of outrage among the very people who are otherwise most critical of Washington and its ways. To these critics, it is the very symbol of congressional arrogance of power, isolation from reality, contempt for the voters, and so on, and demonstrates the need for term limits if not lynching. Consider the good-government advantages of (let's call it) the Byrd Migration. What better way to symbolize an end to the old ways and commitment to reform than physically moving the government? What better way to break up old bureaucracies than to uproot and transplant them, files and all? Second, spreading the government around a bit ought to reduce that self-feeding and self regarding Beltway culture that Washington-phobes claim to dislike so much. Of course there is a good deal of hypocrisy in this anti-Washington chatter. Much of it comes from politicians and journalists who have spent most of their adult lives in Washington and wouldn't care to live anywhere else. They are not rushing to West Virginia themselves, except for the occasional quaint rustic weekend. But they can take comfort that public servants at the Bureau of the Public Debt, at least, have escaped the perils of inside-the-Beltway insularity. Third, is Senator Byrd's raw spread-the-wealth philosophy' completely illegitimate? The Federal Government and government-related private enterprises have made metropolitan Washington one of the richest areas of the country. By contrast, West Virginia is the second poorest state, after Mississippi. The entire country's taxes support the government. Why shouldn't more of the country get a piece of it? As private businesses are discovering, the electronic revolution is making it less and less necessary for work to be centralized at headquarters. There's no reason the government shouldn't take more advantage of this trend as well. It is hardly enough, though, to expel a few thousand midlevel bureaucrats from the alleged Eden inside the Washington Beltway. Really purging the Washington Culture enough to satisfy its noisiest critics will require a mass exodus on the order of what the Khmer Rouge instituted when they took over Phnom Penh in 1975. Until the very members of the TIME Washington bureau itself are traipsing south along 1-95, their word processors strapped to their backs, the nation cannot rest easy. But America's would-be Khmer Rouge should give Senator Byrd more credit for showing the way.

单选题假设二维随机变量(X，Y)服从参数为μ1，μ2，，ρ的正态分布，如果-1＜ρ＜0，则X与Y的协方差矩阵是______．

单选题将一枚硬币连掷5次，如果出现k次正面的概率等于出现k+1次正面的概率，那么k的值为______ A.1 B.3 C.2 D.4 E.5

单选题每只蜘蛛有8条腿，蜻蜓有6条腿和2对翅膀，蝉有6条腿和1对翅膀，现在这3种小虫共有18只，共有118条腿和20对翅膀，其中蝉的数量为( )。 (A)5 (B)6 (C)7 (D)8 (E)9

单选题设则在点O(0，0)处，______

单选题当x→0时，x2-sin x是x的( )。