单选题x5·3yz=7850，其中x5表示十位数是x，个位数是5的两位数；3yz表示百位数是3，十位数是y，个位数是z的三位数，那么：xy=( )．

单选题[2012年10月]某次乒乓球单打比赛中，先将8名选手等分为2组进行小组单循环赛。若一位选手只打了1场比赛后因故退赛，则小组赛的实际比赛场数是( )。

单选题椭mx2+ny2=1与盲线y=-x+1相交于A，B两点，讨原点和线段AB中点的直线斜率为则的值是()．

单选题长途汽车从A站出发，匀速行驶，1h后突然发生故障，车速降低了40%，到B站终点延误达3h，若汽车能多跑50km后，才发生故障，坚持行驶到B站能少延误1h20min，那么A、B两地相距( )km． (A) 412.5 (B) 152.5 (C) 146.5 (D) 137.5

单选题设线性方程组有非零解，则组成基础解系的线性无关的解向量有______．A．1个B．2个C．3个D．4个

单选题President Bush takes to the bully pulpit to deliver a stern lecture to America's business elite. The Justice Dept. stuns the accounting profession by filing a criminal indictment of Arthur Andersen LLP for destroying documents related to its audits of Enron Corp. On Capitol Hill, some congressional panels push on with biased hearings on Enron's collapse and, now, another busted New Economy star, telecom's Global Crossing. Lawmakers sign on to new bills aimed at tightening oversight of everything from pensions and accounting to executive pay. To any spectators, it would be easy to conclude that the winds of change are sweeping Corporate America, led by George W. Bush, who ran as "a reformer with result." But far from deconstructing the corporate world brick by brick into something cleaner, sparer, and stronger, Bush aides and many legislators are preparing modest legislative and administrative reforms. Instead of an overhaul, Bush's team is counting on its enforcers, Justice and a newly empowered Securities & Exchange Commission, to make examples of the most egregious offenders. The idea is that business will quickly get the message and clean up its own act. Why won't the outraged rhetoric result in more changes? For starters, the Bush Administration warns that any rush to legislate corporate behavior could produce a raft of flawed bills that raise costs without halting abuses. Business has striven to drive the point home with an intense lobbying blitz that has convinced many lawmakers that over-regulation could startle the stock market and perhaps endanger the nascent economic recovery. All this sets the stage for Washington to get busy with predictably modest results. A surge of caution is sweeping would-be reformers on the Hill. "They know they don't want to make a big mistake," says Jerry J. Jasinowski, president of the National Association of Manufacturers. That go-slow approach suits the White House. Aides say the President, while personally disgusted by Enron's sellout of its pensioners, is reluctant to embrace new sanctions that frustrate even law-abiding corporations and create a litigation bonanza for trial lawyers. Instead, the White House will push for narrowly targeted action, most of it carried out by the SEC, the Treasury Dept. , and the Labor Dept. The right outcome, Treasury Secretary Paul H. O'Neill said on Mar. 15, "depends on the Congress not legislating things that are over the top." To O'Neill and Bush, that means enforcing current laws before passing too many new ones. Nowhere is that stance clearer than in the Andersen indictment. So the Bush Administration left the decision to Justice Dept. prosecutors rather than White House political operatives or their reformist fellows at the SEC.

单选题设随机变量X和Y相互独立同分布，其分布函数和概率密度分别为F(x)和f(x)，则随机变量Z=min{X，Y)的概率密度为

单选题设S n =1-2+3-4+…+(-1) n-1 n，则S 40 +S 21 +S 23 的值为______

单选题已知a、b∈R，ab＜0，则____。

单选题下列不定积分计算正确的是 ( )

单选题幂级数收敛域是A.(0，2]．B.[0，2)．C.(0，2)．D.[0，2]．

单选题设函数f(x)=cosx，则A．1B．0C．D．-1