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请您张开联想的翅膀,观察如图1的正方体,假如用一个平面截它,会出现什么形状的截面图呢? 显然,截面不会是七边或七边以上的多边形,因为截面图的边框即是截平面与立体某一表面的交线.那么,就让我们来探讨一下截面分别为三、四、五、六边形时的一些特点吧. 一、当截面为三角形时,可以证明,此三角形必为锐角三角形,如图2,据余弦定理可知cosα= 同理,cosβ、cosγ都大于0,即此三角形的三内角
Please open the Lenovo’s wings and observe the cube shown in Figure 1. If you use a plane to cut it, what shape of the cross-section will appear? Obviously, the cross-section will not be a seven-sided or seven-sided polygon, because the cross-sectional view The border is the line of intersection between the plane of the cut plane and the surface of the three-dimensional plane. Then, let us explore some of the characteristics when the sections are respectively three, four, five, and six. First, when the section is a triangle, it can be proved The triangle must be an acute triangle, as shown in Figure 2. According to the cosine theorem, cosα= is the same, cosβ and cosγ are all greater than 0, which is the inner angle of this triangle.