在△ABC中，AB＝10，AC＝6，AD是BC边上的中线，则AD的取值范围是 ．
4＜AD＜16。
延长AD至E，使DE=AD，连接CE．根据SAS证明△ABD≌△ECD，得CE=AB，再根据三角形的三边关系即可求解：
延长AD至E，使DE=AD，连接CE。
∵BD=CD，∠ADB=∠EDC，AD=DE，∴△ABD≌△ECD（SAS）。
∴CE=AB。
在△ACE中，CE－AC＜AE＜CE＋AC，即4＜2AD＜16。
∴4＜AD＜16。
Passage Three
| If the population of the Earth goes on increasing at its present rate, there will eventually not be enough resources left to sustain life on the planet. By the middle of the 21 century, if present trends continue, we will have used up all the oil that drives our cars, for example. Even if scientists develop new ways of feeding the human race, the crowded conditions on Earth will make it necessary for us to look for open space somewhere else. But none of the other planets in our solar system are capable of supporting life at present. One possible solution to the problem, however, has recently been suggested by an American scientist, Professor Carl Sagan. Sagan believes that before the Earth’s resources are completely exhausted, it will be possible to change the atmosphere of Venus and so create a new world almost as large as Earth itself. The difficulty is that Venus is much hotter than the Earth and there is only a tiny amount of water there. Sagan proposed that algae, organisms that can live in extremely hot or cold atmospheres and at the same time produce oxygen, should be bred in conditions similar to those on Venus. As soon as this has been done, the algae will be placed in small rockets. Spaceships will then fly to Venus and fire the rockets into the atmosphere. In a fairly short time, the algae will break down the carbon dioxide into oxygen and carbon. When the algae have done their works, the atmosphere will become cooler but before man can set foot on Venus, it will be necessary for the oxygen to produce rain. The surface of the planet will still be too hot for men to land on it but the rain will eventually fall and in a few years something like Earth will be reproduced on Venus. If the experiments are successful, life will become possible there, but it will not be pleasant at first. When they go to Venus, the first colonists will have to take plenty of water with them and get used to days and nights lasting 60 Earth-days. But there will also be some advantages. The colonists will live longer because their hearts will suffer less strain than on Earth. Apart from that, they will be exploiting a new world while those still on Earth are living in closed, uncomfortable conditions. Perhaps it will be the only way to ensure the survival of the human race. |
The first colonists on Venus will find life difficult there because ______.
A．they will suffer from heart strain
B．they will be no water there
C．the day and night will be very long
D．the rainy season lasts a few years