题目

如图，A，B是海面上位于东西方向相距5(3＋)海里的两个 观测点．现位于A点北偏东45°，B点北偏西60°的D点有一 艘轮船发出求救信号位于B点南偏西60°且与B点相距20海 里的C点的救援船立即前往营救，其航行速度为30海里/小时，该救援 船到达D点需要多长时间？ 答案：解：由题意知海里，   在中，由正弦定理得, =（海里），（4分） 又海里， 在中，由余弦定理得      = 30（海里） 则需要的时间（小时）.（4分） 答：救援船到达D点需要1小时.Children have their own rules in playing games. They seldom need a referee(裁判) and rarely trouble to keep scores. They don’t care much about who wins or loses, and it doesn’t seem to worry them if the game is not finished. Yet, they like games that depend a lot on luck, so that their personal abilities cannot be directly compare. They also enjoyed games that move in stages, in which each stage, the choosing of leaders, the picking-up of sides, or the determining of which side shall start, is almost a game in itself. Grown-ups can hardly find children’s game exciting, and they often feel puzzled at why their kids play such simple game again and again. However, it is found that a child plays games for very important reasons. He can be a good player without having to think whether he is a popular person, and he can find himself being a useful partner to someone of whom he is ordinary afraid. He becomes a leader when it comes to his turn. He can be confident, too, in particular games, that it is his place to give orders, to pretend to be dead, to throw a ball actually at someone, or to kiss someone he has caught. It appears to us that when children play a game they imagine a situation under their control. Everyone knows the rules, and more importantly, everyone plays according to the rules. Those rules may be childish, but they make sure that every child has a chance to win. 40．What is true about children when they play games? 　 A. They can stop playing any time they like.　 B. They can test their personal abilities. 　 C. They want to pick a better team.　　　　 D. They don’t need rules. 41．To become a leader in a game the child has to _____. 　 A. play　　　　　　　　　　 B. wait for his turn 　 C. be confident in himself　　 D. be popular among his playmates 42．What do we know about grown-ups? 　 A. They are not interested in games　 B. They find children’s games too easy 　 C. They don’t need a reason to play games 　 D. They don’t understand children’s games 43． Why does a child like playing games? 　 A. Because he can be someone other than himself. 　 B. Because he can become popular among friends. 　 C. Because he finds he is always lucky in games. 　 D. Because he likes the place where he plays a game. 44．The writer believes that _____. 　 A. children should make better rules for their games 　 B. children should invite grown-ups to play with them 　 C. children’s games can do them a lot of good D. children play games without reasons