The formula of included angle between plane and plane: cosθ=(m*n)/|m||n|. Mathematically, the minimum positive angle formed by the intersection of two straight lines (or vectors) is called the included angle between two straight lines (or vectors), which is usually recorded as ∠ θ (including danger), and the interval range of the included angle between two straight lines is {θ| 0 ≤θ≤π/2}, and the interval range of the included angle between two vectors is {θ| 0 ≤θ≤π}.

Plane means that the whole line connecting any two points on the plane falls on this plane, which is a two-dimensional zero curvature continuation. Such a plane is a straight line that intersects any intersection line of similar surfaces. It is a mathematical concept abstracted from the physical objects that represent life (such as mirrors and calm water), but it is essentially different from these physical objects. It has infinite ductility (that is to say, the plane has no boundary), and there is no difference in size, width and thickness. This property of plane is related to the infinite extension of straight line.