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问题 填空题

已知圆柱的体积为16π cm3，则当底面半径r=______cm时，圆柱的表面积最小．

答案

设圆柱的高为hcm，则

∵圆柱的体积为16π cm3，

∴πr2h=16π，

∴h=

16
r2
，

∴圆柱的表面积S=2πrh+2πr2=

32π
r
+2πr2=
16π
r
+
16π
r
+2πr2≥3
3
16π
r
•
16π
r
•2πr2
=24π，

当且仅当

16π
r
=2πr2，即r=2cm时，取等号，

∴当底面半径r=2cm时，圆柱的表面积最小，最小为24π．

故答案为：2．

选择题

9．近年A、B两国加强资源跨区域调配，据图7判断所调配的资源可能是   

A．水资源  B．木材   C．石油   D．煤炭
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单项选择题

When a disease of epidemic proportions threatens the public, scientists immediately get to work, trying to locate the source of affliction and find ways to combat. Vaccination is one of the effective ways to protect the (1) population of a region or country which may be (2) grave risk. The process of vaccination allows the patient’s body to (3) immunity to the virus or disease so that, if it is encountered, one can fight it (4) naturally. To accomplish this, a small weak or dead (5) of the disease is actually injected into the patient in a controlled environment, (6) his body’s immune system can learn to fight the invader (7) . Information (8) how to penetrate the disease’s defenses is (9) to all elements of the patient’s immune system in a process that occurs naturally, in which genetic information is passed from cell to cell. This makes sure that (10) the patient later come into contact with the real problem, his body is well equipped and trained to (11) with it, having already done so before.

There are, however, dangers (12) in the process. (13) , even the weakened version of the disease contained in the vaccine proves (14) much for the body to handle, resulting in the immune system (15) , and, therefore, the patient’s death. Such is the case of the smallpox vaccine, (16) to eradicate the smallpox epidemic that nearly (17) the whole Native American population and killed massive numbers of settlers. (18) 1 in 10,000 people who receive the vaccine (19) the smallpox disease from the vaccine itself and dies from it. Consequently, the process, which is truly a (20) , may indeed hide some hidden curses.

14（）

A.so

B.too

C.rather

D.quite
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