Axiom: If two planes have a common point, then they have other common points, and the set of these common points is a straight line passing through this common point.

Plane and straight line

1, point a is in plane α, and it is marked as a ∈ α; Point b is not in the plane α, and is recorded as b? α。

2. the point p is on the straight line l, and it is marked as p ∈ l; Point p is outside the straight line l, and it is recorded as p? Me.

3. If all the points on the straight line L are in the plane α, it is said that the straight line L is in the plane α, or the plane α passes through the straight line L, which is called L? α, otherwise the straight line L comes out of the plane α, and it is recorded as L? α。

4. Plane α and β intersect on a straight line L, which is denoted as α ∩ β = L.

5. Is the straight line A recorded as a in the plane α? α。