单选题已知等差数列{a n }中，a 7 +a 9 =16，a 4 =1，则a 12 的值是______

单选题方程的根的个数是()．A．0B．1C．2D．3

单选题设x1和x2是方程2x2-4x+5=0的两个根，则|x1—x2|=()。A．0B．C．D．不存在

单选题设P(A)＞0，P(B)＞0，且A与B二事件互斥，下列关系式正确的是______A．P(B)=P(B|A)．B．C．D．P(B)=1-P(A)．

单选题设，B是2阶矩阵，且满足AB=B，k1，k2是任意常数，则B=

单选题The unfortunate death of the genius poet caused ______ loss to this country. A.priceless B.countless C.incalculable D.imaginable

单选题长度是800米的队伍的行军速度为100米/分钟，在队尾的某人以3倍于行军的速度赶到排头，并立即返回队尾所用的时间为( )分钟．

单选题从这20个自然数中任取3个不同的数，使它们成等差数列，这样的等差数列共有______。

单选题已知(x+i)3是一个实数，则实数x=______。A．B．C．D．

单选题(x 3 +3x 2 +18x-1)(6x 3 +x 2 +2x+3)的展开式中，x 3 的系数是( )．

单选题某种商品按原价出售，每件利润是成本的，后来打9折出售，每天的销售量翻了一番，这种商品打折后每天总利润比打折前增加了______。

单选题不等式x2-2x0的解集为（　）

单选题Attempts to understand the relationship between social behavior and health have their origin in history. Dubos (1969) suggested that primitive humans were closer to the animals (1) they, too, relied'upon their instincts to stay healthy. Yet some primitive humans (2) a cause and effect relationship between doing certain things and alleviating (3) of a disease or (4) the condition of a wound. (5) there was so much that primitive humans did not (6) the functioning of the body, magic became an integral component ofthe beliefs about the causes and cures of heath (7) Therefore it is not (8) that early humans thought that illness was caused (9) evil spirit. Primitive medicines made from vegetables or animals were invariably used in combination with some form of ritual to (10) harmful spirit from a diseased body. One of the. earliest (11) in the Western world to formulate principles of health care based upon rational thought and (12) of supernatural phenomena is found in the work of the Greek physician Hippocrates. The writing (13) to him has provided a number of principles underiying modern medical practice. One of his most famous (14) , the Hippocratic Oath, is the foundation of contemporary medical ethics. Hippocrates also argued that medical knowledge should be derived from a (15) of the natural science and the logic of cause and effect relationships. In this (16) thesis, On Air, Water, and Places, Hippocrates pointed out that human well-being is (17) by the totality of environmental (18) : living habits or lifestyle, climate, geography of the land, and the quality of air, and food. (19) enough, concerns about our health and the quality of air, water, and places are (20) very much written in twentieth century.

单选题已知函数f(x)满足,则f(x0)=

单选题设矩阵，则下列矩阵中与A相似的为

单选题设z=ex+y；则dz|(1,1)=（）

单选题二次积分等于()．

单选题设A是m×n矩阵，r

单选题设f(x)为单调可微函数，g(x)与f(x)互为反函数，且f(2)=4，f"(2)=，f"(4)=6，则g"(4)等于______．A．B．C．D．4

单选题给出下列两个命题：①如果一条直线与一个平面垂直，则该直线与该平面内的任意一条直线垂直②以二面角的棱上任意一点为端点，在二面角的两个面内分别作射线，则这两条射线所成的角为该二面角的平面角则(　　)