1: Judgment Theorem: The straight line L is perpendicular to m, the straight line L is perpendicular to n, the straight lines M and N are on the plane A, and the straight lines M and N have the intersection point P, so the straight line L is perpendicular to the plane A..

2. the nature of vertical plane: plane a is perpendicular to plane b, plane a and plane b have intersection line c, line d is on plane b, and line d is perpendicular to plane a.

Prove that the surface is vertical:

1: Judgment Theorem: If the straight line L is perpendicular to B and the straight line L is on the surface A, then the surface A is perpendicular to the surface B..

2. Calculate the dihedral angle equal to 90 degrees.