If the normal vectors of two faces of a dihedral angle are m = (0,0,3) and N = (8,9,2), then the cosine of the dihedral angle is?

If the normal vectors of two faces of a dihedral angle are m = (0,0,3) and N = (8,9,2), then the cosine of the dihedral angle is?

|m|=3
|n|=√(8²+9²+2²)=√149
m.n=6
cos=m.n/|m||n|=6/(3*√149)=2√149/149
So the cosine of dihedral angle is arccos (2 √ 149 / 149) or π - arccos (2 √ 149 / 149)

When calculating dihedral angle with space vector method, if cosine cos α of angle between two normal vectors is negative, how to express α
If cos α = - 7 / 8, one l and two normal vectors are inward at the same time, how can α be expressed in the formula? One of the two normal vectors refers to the inside and the other refers to the outside, how can α be expressed in the formula? We are going to take the college entrance examination on the 7th,

cosα=－7／8,a=arccos﹙-7/8﹚=π-arccos﹙7/8﹚.
One l two normal vectors simultaneously inward or outward: π - A
Two two normal vectors, one inside and one outside, are: a

How to prove that a line vector is parallel to a plane

Find out the normal vector of the plane, which is perpendicular to the line, and prove that the line vector is parallel to the plane