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实际中有一些结构,在某一个方向上尺寸是不变的(如平板结构),并且所有与这个方向垂直的截面,均为完全相同的平面图形,而到两侧平面距离相等的截面称之为中面(见图1)。
There are actually some structures that are dimensionally constant in one direction (such as a flat plate structure) and all the sections perpendicular to this direction are exactly the same plane and the cross-section equidistant from the plane on both sides is called For the middle (see Figure 1).