28.7.3 Dihedral Angles
Core ideaThe angle an open book makes: draw a perpendicular into each page from the spine, and the angle you measure is the dihedral angle.
Module goal. Master the tests and properties of perpendicularity for line-to-line, line-and-plane, and plane-and-plane along with the key angles, completing the pure-geometry approach and connecting to the vector method ahead.