单选题=______A．∞B．0C．1D

单选题曲线的凸弧区间是______。A.(-∞，-8)B.(-8，-4)C.(-4，4)D.(4，+∞)

单选题设向量β可由向量组α1，α2，…，αm线性表示，但不能由向量组(Ⅰ)：α1，α2，…，αm-1线性表示.记向量组(Ⅱ)：α1，…，αm-1，β，则 (A)αm不能由(Ⅰ)线性表示，也不能由(Ⅱ)线性表示. (B)αm不能由(Ⅰ)线性表示，但可由(Ⅱ)线性表示. (C)αm可由(Ⅰ)线性表示，也可由(Ⅱ)线性表示. (D)am可由(Ⅰ)线性表示，但不能由(Ⅱ)线性表示. [ ]

单选题函数y=ln(1+x 2 )的单调增区间是

单选题设A是n阶矩阵，α是n维列向量．若秩，则线性方程组______．A．AX=α必有无穷多解B．AX=α必有唯一解

单选题如图所示，在直角三角形ABC中，∠C=90°，AF=FE=ED=DC=CB，则∠A=()。

单选题设向量组S={a1，a2，a3}线性无关，下列向量组中，与S等价的有( )． ①a1-a3，a2-a3 ②a1，a1+a2，a1+a2+a3 ③a1-a3，a1+a3，2a1，3a3 ④a1-a3，a1+a3，2a2，3a3

单选题已知方程x2+ax+b=0的两实根之比为3:4，判别式Δ=2，则两个实根之差的绝对值为______。A．B．C．D．

单选题行列式=A.(ad-bc)2．B.-(ad-bc)2．C.a2d2-b2c2．D.b2c2-a2d2．

单选题The marvelous telephone and television network that has now enmeshed the whole world, making all men neighbors, cannot be extended into space. It will never be possible to converse with anyone on another planet. Even with today's radio equipment, the messages will take minutes--sometimes hours--on their journey, because radio and light waves travel at the same limited speed of 186, 000 miles a second. Twenty years from now you will be able to listen to a friend on Mars, but the words you hear will have left his mouth at least three minutes earlier, and your reply will take a corresponding time to reach him. In such circumstances, an exchange of verbal messages is possible--but not a conversation. To a culture which has come to take instantaneous communication for granted, as part of the very structure of civilized life, this "time barrier" may have a profound psychological impact. It will be a perpetual reminder of universal laws and limitations against which not all our technology can ever prevail. For it seems as certain as anything can be that no signal--still less any material object--can ever travel faster than light. The velocity of light is the ultimate speed limit, being part of the very structure of space and time. Within the narrow confines of the solar system, it will not handicap us too severely. At the worst, these will amount to twenty hours--the time it takes a radio signal to span the orbit of Pluto, the outer most planet. It is when we move out beyond the confines of the solar system that we come face to face with an altogether new group of cosmic reality. Even today, many otherwise educated men--like those savages who can count to three but lump together all numbers beyond four--cannot grasp the profound distinction between solar and stellar space. The first is the space enclosing our neighboring worlds, the planets, the second is that which embraces those distant suns, the stars, and it is literally millions of times greater. There is not such abrupt change of scale in the terrestrial affairs. Many conservative scientists, shocked by these cosmic gulfs, have denied that they can ever be crossed. Some people never learn, those who sixty years ago laughed at the possibility of flight, and ten years ago laughed at the idea of travel to the planets, are now quite sure that the stars will always be beyond our reach. And again they are wrong, for they have failed to grasp the great lesson of our age--that if something is possible in theory, and no fundamental scientific laws oppose its realization, then sooner or later it will be achieved. One day we shall discover a really efficient means of propelling our space vehicles. Every technical device is always developed to its limit and the ultimate speed for spaceships is the velocity of light. They will never reach that goal, but they will get very near it. And then the nearest star will be less than five years of voyaging from the earth.

单选题一件工程要在规定时间内完成，若甲单独做要比规定的时间推迟4天，若乙单独做要比规定的时间提前2天完成。若甲、乙合作了3天，剩下的部分由甲单独做，恰好在规定时间内完成，则规定时间为( )天。 (A) 19 (B) 20 (C) 21 (D) 22 (E) 24