单选题微分方程(2x-y)dx+(2y-x)dy=0的通解为______。(C为任意常数) A.x2+xy+y2=C B.x2-xy+y2=C C.2x2-xy+3y2=C D.2x2+xy+3y2=C

单选题函数在x=0处连续，则a=______A.1B.2C.3D.4

单选题已知函数F(x)是f(x)的一个原函数，则不定积分∫f(2x)dx=______A．B．C．F(x)+CD．F(2x)+C

单选题设则下面四个命题①f(x)在[-1，1]上存在原函数②g(x)在[-1，1]上存在原函数③存在定积分④g'(0)存在中正确的是

单选题 It is hard to predict how science is going to turn out, and if it is really good science it is impossible to predict. If the things to be found are actually new, they are by definition unknown in advance. You cannot make choice in that matter. You either have science or you don't have, and if you have it you are obliged to accept the surprising and disturbing pieces of information, along with the neat and promptly useful bits. The only solid piece of scientific truth about which I feel totally confident is that we are profoundly ignorant about nature. I regard this as the maj or discovery of the past hundred years of biology. It is, in its way, an illuminating piece of news. It would have amazed the brightest minds of the 18th century Enlightenment to be told by any of us how little we known and how bewildering seems the way ahead. It is this sudden confrontation with the depth and scope of ignorance that represents the most significant contribution of the 20th century science to the human intellect. In earlier times, we either pretended to understand how things worked or ignored the problem, or simply make up stories to fill the gaps. Now that we have begun exploring in eamest, we are getting glimpses of how huge the questions are, and how far they are from being answered. Because of this, we are depressed. It is not so bad being ignorant if you are totally ignorant. The hard thing is knowing in some detail the reality of ignorance, the worst spots and here and there the not-so-bad spots, but no true light at the end of the tunnel nor even any tunnels that can yet be trusted. But we are making a beginning, and there ought to be some satisfaction. There are probably no questions we can think up that can't be answered, sooner or later, including even the matter of consciousness. To be sure, there may well be questions we can't think up ever, and therefore limits to the reach of human intellect, but that is another matter. Within our limits, we should be able to work our way through to all our answers, if we keep at it long enough, and pay attention.

单选题已知圆O：(x-3)2+(y+5)2=36和点A(2，2)，B(-1，-2)，若点C在圆上且△ABC的面积为，则满足条件的点C的个数是______．

单选题若，则=______．

单选题设f(x)在(-∞，+∞)内有定义，下列函数中不是偶函数的有______。 A．y=c B．y=f(x2) C．y=f(x)+f(-x) D．y=|f(x)|

单选题正项级数收敛是级数收敛的

单选题A car dealer who gives a customer a 20 percent discount on the list price of a car still realizes a net profit of 25 percent of cost. If the dealer's cost is $4800, what is the usual list price of the car? (A) $6000 (B) $6180 (C) $7200 (D) $7500 (E) $8001

单选题方程x2y"+2xy"-2y=0的通解为A．y=C1ex+C2e2x．B．y=(C1+C2x)ex．C．y=C1x+C2x2．D．

单选题实数x，y，z满足，则(4x-10y)z等于______.A．B．C．D．E．