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有一种特殊的四面体,它的同一顶点上的3条棱两两垂直,我们不妨将其称之为直四面体,含直角的面称为直角面,不含直角的面称为斜面.
There is a special tetrahedron whose three vertices on the same vertex are perpendicular to each other. Let us call it a straight tetrahedron. A face with right angles is called a right angle face, and a face without a right angle is called a slope.