Well is really simpler than that. The angle between two planes is the angle between their normals.Angle between 2 planes. We are taught to locate such an angle with 2 vertices that descend towards the intersection of the planes and the vertex of it. But what is the geometric proof that all the angles we create like this are the same ?!
No you explained yourself. You just do not know or don't understand the definition.I probably didn't explain myself well. What guarantees / what is the proof, that all required angles, between two levels, are the same / the same size?
But two lines that share a point, define a plane. Just work in that plane. No need to bother with space or any other plane.The definition in high school: It is an angular definition of the method of downloading 2 verticals to the same point, on the intersection of the 2 planes, in each of the planes separately, as I have painted several times in our discussion.
This is easy to prove, if it were two angles on the same plane (simple geometry)