单选题设A为n阶非零矩阵，E为n阶单位矩阵，若A3=O，则 (A)E-A不可逆，E+A不可逆. (B)E-A不可逆，E+A可逆. (C)E-A可逆，E+A可逆. (D)E-A可逆，E+A不可逆. [ ]

单选题烧杯中有1L纯酒精，倒出一定量的酒精后，再用水补充至原体积，然后再倒出和第一次一样多的酒精溶液，再用水补充至原体积，此时烧杯中酒精溶液的浓度变为64%，则两次共补充水的体积为______．

单选题设A是三阶矩阵，其中a11≠0，Aij=aij，i=1，2，3，j=1，2，3，则|2AT|= A.0． B.2． C.4． D.8．

单选题与矩阵合同的矩阵是

单选题设O为坐标原点，A、B在抛物线y2=2x上，OA⊥OB，且|OA|=|OB|，则△ABC的面积是( )．

单选题设f(x)连续，，则F"(x)等于

单选题These are tough times for Wal-Mart, America's biggest retailer. Long accused of (1) small-town America mad condemned for the selfishness of its pay, the company has lately come under (2) for its meanness over employees' health-care benefits. The charge is arguably (3) : the firm's health coverage, while (4) less extensive than the average for big companies, is on equal terms (5) other retailers'. But bad publicity, coupled with rising costs, has (6) the Bentonville giant to action. WalMart is making changes that should shift the ground in America's healthcare debate. One (7) is to reduce the prices of many generic, or out-of-patent, prescription drugs. Wal-Mart's critics dismiss the move as a publicity (8) . The list of drugs includes only 143 different medicines and excludes many popular group. True, but short-sighted. Wal-Mart has (9) retailing by using its size to squeeze suppliers and (10) the gains on to consumers. It could (11) the same with drugs. A "Wal-Mart effect" in drugs will not solve America's health-costs problem: group account for only a small share of drug costs, which in turn make (12) only 10% of overall health spending. But it would (13) . The firm's other initiative is more (14) . Wal-Mart is joining the small but fast-growing group of employers (15) are controlling costs by shifting to health insurance with high deductibles. Early evidence (16) these plans do help firms control the cost of health insurance. But critics say that the savings are (17) . They argue that the plans shift costs to sicker workers, discourage preventative care and will anyway do little to (18) overall health spending, (19) most of the $2 trillion that America (20) health care each year goes to people with multiple chronic diseases.

单选题设，则A-1=A．B．C．D．

单选题假设甲、乙两国关于拥有洲际导弹数量的关系曲线y=f(x)和x=g(y)的意义是： 当甲国拥有导弹x枚时，乙国至少需储备导弹y=f(x)枚，才有安全感； 当乙国拥有导弹y枚时，甲国至少需储备导弹x=g(y)枚，才有安全感． 这两条曲线将坐标平面的第一象限分成四个 区域Ⅰ，Ⅱ，Ⅲ，Ⅳ，如图图5-1所示．双方均有安全感的区域是( )．

单选题设f(x，y)在点P 0 (x 0 ，y 0 )的某邻域内有连续的二阶偏导数

单选题把浓度为50%的酒精溶液90千克全部稀释为浓度为30%的酒精溶液，需要加水______千克。 A．60 B．70 C．85 D．105

单选题设A，B为实对称矩阵，则A合同于B，如果

单选题设为______．A．B．C．D．

单选题()A．绝对收敛B．条件收敛C．发散D.收敛性与口有关

单选题下列说法中错误的是______(设D(X)≠0，D(Y)≠0)．