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在介绍这个新方法之前,我们先看下面一个简单实验。如图1,a、b、c为正方体(1)共顶点的三个面,将正方体(1)按箭头上方的文字说明进行转动。本文中的转动专指翻转和旋转,翻转又指向前或向后或向左或向右翻转90°,旋转又指按顺时针或逆时针方向水平旋转90°。由图可知,(3)、(5)、(7)中的a、b、c与(1)中的a、b、c处在同一个观察角度。由(1)到(3)、(5),a的方位改变了,b、c的方位也改变了;由(1)到(7),a的方位未改变,b、c的方位也未改变。
Before introducing this new method, let’s look at a simple experiment below. As shown in Figure 1, a, b and c are the three faces of the cube (1), and the cube (1) is rotated according to the text above the arrow. Turning in this article specifically refers to turning and turning, turning and pointing forwards or backwards or left or right turns 90 °, and rotation refers to rotating horizontally 90 ° clockwise or counterclockwise. It can be seen from the figure that a, b and c in (3), (5) and (7) are at the same viewing angle as a, b and c in (1). From (1) to (3), (5), the orientation of a has changed, and the orientation of b and c has also changed. From (1) to (7), the orientation of a has not changed and the orientation of b and c has not change.