•
Yours:
•
wangwen
• The city stands on the bank of the Huanghe River. It is a beautiful place for people to live in. Its economy has been developing rapidly in the past ten years. New factories, houses and roads have been built. More schools and hospitals are available for its people. However, there are still some problems, such as water and air pollution and heavy traffic in rush hours. In my opinion, Zhengzhou should develop its economy scientifically; I would also think that the growth of its population should be brought under control so that we’ll have a better hometown in future.
B′ A
A′ O
C′A′，则△A′B′C′ 为所求.
C′
(1)如图，已知△ABC 与△DEF 中心对称， 点 A 和点 D 是对称点，画出对称中心 O．
解：连接AD，CF，F
E
AD与CF交于点O，A
则点O为所求.
O
D
B
C
(2)如图，以顶点 A 为对称中心，画一个与已知四边 形 ABCD 成中心对称的图形．
第25课时23.2中心对称(1)
(1)如图，把其中一个图案绕点 O 旋转180°， 你有什么发现？
两个图案能够完全重合在一起．
O
(2)如图，线段 AC，BD 相交于点 O，OA=OC，
OB=OD．把 △OCD 绕点 O 旋转 180°，你有什么
发现？
A
D
O
B
C
两个图案能够完全重合在一起．
中心对称是特殊的旋转，它有哪些性质？
C
A
B
O B′
A′
C＇
(1)点 O 在线段 AA＇上吗？如果在，在什么位置？ (2)△AOB 和△A′OB′有什么关系？
(3)你能从这个探究中得到什么结论？
B′ A
O
A′
B
中心对称的性质 (1)中心对称的两个图形， 对称点所连线段都 经过对称中心，而且被对称中心所平分； (2)中心对称的两个图形是全等图形．
• Dear Jack,
• It’s very kind of you to write me and let me know about your beautiful city. Now I’d like to tell you something about my hometown Zhengzhou.
1.连接OA， 2.延长AO，
O A＇
在AO的延长线上截取OA＇，
使OA＇= OA. 则点A＇为所求.
例1 (2)如右图，选择点 O 为对称中心，画
出与△ABC关于点 O 对称的△A′B′C′ ． C
画法：
1.分别作点A，B，C
关于点O为对称中心 的对称点A′、B′、C′.
B
2.连接 A′B′、B′C′、
opinion, Zhengzhou should develop its economy scientifically; I would also think that the growth of its population should be brought under control so that we’ll have a better hometown in future.
B′ A
O A′
B
例1 (1)如左图，选择点 O 为对称中心，画
出点 A 关于点 O 的对称点 A＇；
(2)如右图，选择点 O 为对称中心，画出与
△ABC关于点 O 对称的△A′B′C′ ． C
A O
A
B
O
例1 (1)如左图，选择点 O 为对称中心，画
出点 A 关于点 O 的对称点 A＇；
画法： A
New factories, houses and roads have been built. More schools and hospitals are available for its people. However, there are still some problems, such as water and air pollution and heavy traffic in rush hours. In my
• The city stands on the bank of the Huanghe River. It is a beautiful place for people to live in. Its economy has been developing rapidly in the past ten years.
D
C
B′
A
B
c′
四边形AB′C′D′ 为所求．
D′
小结
(1)本节课学了哪些主要内容？ (2)怎样画一个图形关于一个点的对称图形？
6．布置作业
教科书第 66 页，练习 1，2 题．
课件说明
• 本节课从旋转变换引入中心对称的概念，先让学生从 旋转的角度观察两个图形之间的关系，类比旋转得出 中心对称的定义，渗透了从一般到特殊的思想方法． 在此基础上，通过探究成中心对称的两个图形的对称 中心与对应点所连线段之间的关系得到中心对称的性 质，并能运用中心对称的性质画出一个图形关于某一 点中心对称的对称图形．
课件说明
• 学习目标： 1．知道中心对称的概念，能正确表述中心对称的性 质； 2．会画一个图形关于某一点中心对称的对称图形．
• 学习重点： 中心对称的概念和性质．
•
• Nov .10 • 书面表达
• Dear Jack,
• It’s very kind of you to write me and let me know about your beautiful city. Now I’d like to tell you something about my hometown Zhengzhou.