单选题设{a n }是等比数列，S n 是它的前n项和，若S n =10，S 2n =30，则S 6n -S 5n 等于( )．

单选题设f(x)在x=0处可导，，则f′(0)=()．

单选题设为连续函数，则a等于______

单选题____A．1B．cos1C．0D．

单选题 分别是数域上的维线性空间，则由全体双线性型组成的线性空间的维数是 A、 B、 C、 D、

单选题接种某疫苗后，出现发热反应的概率为0.80．现有5人接种该疫苗，至少有3人出现发热反应的概率为(精确到0.01)______

单选题在由1、2、3、4、5构成的各位数字不同的三位数中，任取一个恰是偶数的概率为{{U}}{{/U}}。A．B．C．D．

单选题等于____。A．0B．C．2D．1

单选题Arsenic, a naturally occurring poison and carcinogen found in ground water, is strongly linked to adult-onset diabetes, U. S. researchers said on Tuesday. Odorless, tasteless, colorless and easily soluble in water or wine, arsenic has long been a feared poison. A heavy dose is detectable in a corpse, but researchers say small amounts of arsenic may sicken people gradually. Dr. Ana Navas-Aeien and colleagues at Johns Hopkins University in Baltimore found a " relatively strong " association between commonly found levels of arsenic in urine and type 2 diabetes in a study of American adults. " It seems there is maybe no safe level of arsenic. " Navas-Acien said in a telephone interview. " Worldwide it's a huge problem, " she said. " As water becomes a scarce resource. the situation becomes even more serious. " Arsenic raises the risk for cancers of the bladder, lung, kidney, skin and, possibly, the pros tate, Navas-Acien said. The 20 percent of nearly 800 study participants who had the most arsenic in their bodies, a tolerable 16. 5 micrograms per liter of urine, had 3. 6 times the risk of developing late-onset diabetes than those in the bottom 20 percent, who had 3 micrograms per liter. Levels of arsenic were 26 percent higher in people with late-onset, or type 2, diabetes than those without the disease, the study found. The U. S. government sets a limit for drinking water at 10 micrograms of arsenic per liter, which is exceeded in the water consumed by 13 million Americans who mostly live in rural areas that rely on wells to bring up ground water, the researchers wrote in the Journal of the American Medical Association. Arsenic contaminates drinking water for millions of people in Bangladesh, parts of Central Europe, Chile, Argentina and the western United States, where ground water is the source of drinking water and the land has higher concentrations of arsenic. Overall, 7. 8 percent of Americans are believed to have diabetes, although some do not know it. At least 90 percent of cases are the type 2 variety, in which the body loses its ability to use insulin properly. Navas-Acien said arsenic may play a significant role in diabetes incidence, but it is difficult to say how much. Arsenic can accumulate in the body, and can ruin the body's ability to use insulin and perform the vital task of converting blood sugar into energy. Normally, insulin fits into cells via molecular doorways called receptors, which in turn signal the cell to move glucose inside, but arsenic enters the cell and somehow blocks the activity. Seafood is another source of arsenic, but the organic form found in shellfish and some fish has a carbon molecule attached and poses no risk to health, she said.

单选题Aboxcontains100balls,numberedfrom1to100.Ifthreeballsareselectedatrandomandwithreplacementfromthebox,whatistheprobabilitythatthesumofthethreenumbersontheballsselectedfromtheboxwillbeodd?

单选题设函数f(x)连续，，其中区域Duv为图中阴影部分，则______．A．vf(u2)B．C．vf(u)D．

单选题若(4x2-8xy+3y-9)2+|5x+3|=0，则25(8xy-3y)+190的值是( )．

单选题(1)0＜c＜a＜b(2)0＜a＜b＜c

单选题曲线|xy|+1=|x|+|y|所围成的图形的面积为______A．B．C．1D．2E．4

单选题当k为( )时，方程2x 2 -(k+1)x+(k+3)=0的两根之差为1。

单选题设f(x，y)在(0，0)处连续，且，则()

单选题甲乙两人在400m的跑道上参加长跑比赛，甲乙同时出发，甲跑3圈后，第一次遇到乙，如果甲的平均速度比乙的平均速度快3m／s，则乙的平均速度为( )。

单选题求平行直线x+3y+8=0和x+3y-6=0的中位线．

单选题已知矩阵，那么与A既相似又合同的矩阵是______A．B．C．D．

单选题一根铁丝，先截下它的，又截下原长的，结果两根相差0.5米，这根铁丝原来的长度为()米．