The triple perpendicular theorem refers to a straight line on a plane. If it is perpendicular to the projection of a diagonal line passing through this plane on this plane, then it is also perpendicular to this diagonal line. According to the idea of three perpendicular lines theorem, the plane angle of dihedral angle is constructed, and then the calculation method of plane angle of dihedral angle is obtained.

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1, the three perpendicular theorem is one of the important theorems of solid geometry. If a straight line in a plane is perpendicular to the projection of a diagonal line in this plane, it is also perpendicular to the diagonal line. The Three Verticality Theorem determines that the diagonal is perpendicular to the straight line in the plane through the vertical relationship between the projection of the diagonal in the plane and the straight line in the plane. Because it involves three straight lines perpendicular to the known straight lines on the plane, it is called the three vertical theorem.

2. The essence of the Three Verticality Theorem is the judgment theorem that a diagonal line in space is perpendicular to a straight line in the plane. The key to the application of the three perpendicular theorem is to find the perpendicular of the plane (datum). As for projection, it is determined by vertical feet and inclined feet, so it is the second type.

A program to prove a⊥b is obtained from the proof of the theorem of three perpendicular lines: one perpendicular line, two perpendicular lines and three perpendicular lines. That is, the first alignment plane (datum plane) and the second alignment projection line are vertical, then A and B form a straight line and a diagonal line on the plane. Thirdly, it is proved that the projective straight line is perpendicular to the straight line A, thus the conclusion that A is perpendicular to B is drawn. ..

3. Triorthogonal theorem and its inverse theorem, as a typical method to prove problems, are widely used in solving problems. When applying the triple perpendicular theorem, we should not only pay attention to the diversity of the triple perpendicular theorem, but also pay attention to the application of the triple perpendicular theorem in the vertical plane or inclined plane.

According to the principle that "two vertical lines, three diagonal lines and projections will appear in a certain plane", the key to confirm the figure and obtain the proved vertical relationship is to find the projection of the vertical line and diagonal line in the plane.

4. Inverse theorem theorem of three perpendicular lines: If a straight line in a plane is perpendicular to a diagonal line passing through the plane, then the straight line is also perpendicular to the projection of the diagonal line in the plane.