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为了观察自己的仪表是否整洁漂亮,你常常要照镜子．在镜子中,人或物体与其像是完全一样的．我们都有这样的经验:当人走近镜面,人在镜中的像也走进镜面;当人远离镜面,人在镜中的像也远离镜面．如果你留心的话就可以发现:人和像与镜面的距离保持相等(图1),这种现象叫做面对称．如果我们只取一个侧面,那么镜面就可用一条直线来表示,人和像可用一个平面图形来表示,这样,人、像与镜就成了轴对称,也叫直线对称．
In order to observe whether your instrument is clean and beautiful, you often need to look in the mirror. In a mirror, people or objects are exactly the same as they are. We all have this experience: When people approach the mirror, the image of the person in the mirror also goes into the mirror; when the person is far from the mirror, the person’s image in the mirror is also far away from the mirror. If you look closely you can see that the distance between the person and the image is the same as the mirror (Figure 1). This phenomenon is called surface symmetry. If we only take one side, then the mirror can be represented by a straight line, and the person and the image can be represented by a plane figure. Thus, the person, the image, and the mirror become axisymmetric, also called line symmetry.