单选题下列反常积分中，收敛的是()．

单选题 In their world of darkness, it would seem likely that some of the animals might have become blind, as has happened to some cave fauna. So, indeed, many of them have, compensating for the lack of eyes with marvelously developed feelers and long, slender fins and processes with which they grope their way, like so many blind men with canes, their whole knowledge of friends, enemies, or food coming to them through the sense of touch. The last traces of plant life are left behind in the thin upper layer of water for no plant can live below about 600 feet even in very clear water, and few find enough sunlight for their food-manufacturing activities below 200 feet. Since no animal can make its own food, the creatures of the deeper waters live a strange, almost parasitic existence of utter dependence on the upper layers. These hungry carnivores prey fiercely and relentlessly upon each other, yet the whole community is ultimately dependent upon the slow rain of descending food particles from above. The components of this never-ending rain are the dead and dying plants and animals from the surface, or from one of the intermediate layers. For each of the horizontal zones or communities of the sea that lie between the surface and the sea bottom, the food supply is different and in general poorer than for the layer above. Pressure, darkness, and silence are the conditions of life in the deep sea. But we know now that the conception of the sea as a silent place is wholly false. Wide experience with hydrophones and other listening devices for the detection of submarines has proved that, around the shore lines of much of the world, there is the extraordinary uproar produced by fishes, shrimps, porpoises and probably other forms not yet identified. There has been little investigation as yet of sound in the deep, offshore areas, but when the crew of the Atlantis lowered a hydrophone into deep water off Bermuda, they recorded strange mewing sounds, shrieks, and ghostly moans, the sources of which have not been traced. But fish of shallower zones have been captured and confined in aquaria, where their voices have been recorded for comparison with sounds heard at sea, and in many cases satisfactory identification can be made. During the Second World War the hydrophone network set up by the United States Navy to protect the entrance to Chesapeake Bay was temporarily made useless when, in the spring of 1942, the speakers at the surface began to give forth, every evening, a sound described as being like "a pneumatic drill tearing up pavement". The extraneous noises that came over the hydrophones completely masked the sounds of the passage of ships. Eventually it was discovered that the sounds were the voices of fish known as croakers, Which in the spring move into Chesapeake Bay from the offshore wintering grounds. As soon as the noise had been identified and analyzed, it was possible to screen it out with an electric filter, so that once more only the sounds of ships came thorugh the speakers.

单选题设ai(i=1，2，3)皆为正实数，λ1＜λ2＜λ3，则方程

单选题若数列{an}中，an≠0(n≥1)，是()．

单选题数列{an}中，a1=3，a2=7．当n≥1时，an+2等于anan+1的个位数，则a2006= A. 9 B. 7 C.3 D. 1 E. 2

单选题函数f(x)的最小值为6 (1)f(x)=|x-2|+|x+4| (2)f(x)=|x+3|+|x-3| A.条件(1)充分，但条件(2)不充分； B.条件(2)充分，但条件(1)不充分； C.条件(1)和(2)单独都不充分，但条件(1)和(2)联合起来充分； D.条件(1)充分，条件(2)也充分； E.条件(1)和(2)单独都不充分，条件(1)和(2)联合起来也不充分。

单选题若级数和都收敛，则级数

单选题如果x 1 ，x 2 ，x 3 三个数的算术平均值为5，则2x 1 +2，2x 2 -3，2x 3 +6与9的算数平均值是______

单选题在等差数列{an}中，满足3a4＝7d7，且a1＞0，Sn是数列{an}前n项的和．若Sn取得最大值，则n＝( )．

单选题设f(x，y)在点(x0，y0)处两个偏导数fx(x0，y0)，fy(x0，y0)都存在，则A．f(x，y)在(x0，y0)处连续．B．存在．C．D．f(x，y)在(x0，y0)处可微．

单选题如果那么的值为______．

单选题设z=x2+y2，则等于____。

单选题二次曲面x2+2y2+xy-3xz-7x-4y+9=0的中心是( )。 A．(1，2，0) B．(0，1，2) C．(0，2，1) D．(0，1，-2)

单选题4人报名参加3项比赛，每人报且只报1项，则不同的报法有( )种．