Face-to-face relationship basically includes intersection and parallel, and vertical is a special case of intersection.

There are generally two methods to solve the relationship: geometric method and vector method.

Geometric method is brain-consuming, and can generally be considered in cubes or their cutting bodies (such as triangular prisms), so it is good to practice spatial imagination.

Personally, I suggest using vector method to solve problems, and it is also possible to use normal vector n flexibly:

1. When the normal vectors are perpendicular to each other, the planes are perpendicular (n 1 n2=0 = 0).

2. If a normal vector is perpendicular to another surface, the surfaces are parallel (the normal vectors are multiplied by the two cross vectors in the surface respectively =0).

3. The included angle between planes is equal to the included angle of normal vector (COSA = n 1 N2/ modular product).