What are the ways to prove that lines are vertical?

Definition: The angle between two straight lines is 90 degrees.

Three perpendicularity theorem: if a straight line in a plane is perpendicular to its projection in the plane, it is perpendicular to the diagonal line passing through the plane.

Definition of lines and planes: If 1 lines are perpendicular to a plane, then it is perpendicular to all the lines in this plane.

Normal vector: In the spatial rectangular coordinate system, three points and two vectors define a plane, and the vector perpendicular to these two vectors is perpendicular to this plane, that is, the vector whose product of these two vectors is 0 is perpendicular to this plane, that is, the normal vector of this plane.