(1)- Prove that the straight line is parallel to the surface. When the inner surface allows two straight lines to intersect in parallel for the first time, it can be allowed.
(2)- Prove that it is parallel to the surface, first parallel to the plane where the inner surfaces of two intersecting lines are located, and then allow the surface to be parallel to the plane (the line is on the plane).
(3)- Prove that the straight line is perpendicular to the surface and the first clamping line is perpendicular to the two intersecting straight lines.
④ —— Prove that the plane is perpendicular to the surface, the first clamping line is perpendicular to the surface of two intersecting lines, and then allow the surface to be perpendicular to the plane.
I am a student of 1 myself. These are what the teacher told us. I think these formulas are very useful. It's just that when I do these questions, I always say that he wrote them. But to be honest, I should remember the definition of textbooks, which is more problematic and more natural.