Dihedral angle
From AoPSWiki
The dihedral angle is the angle formed by two planes in three dimensional space. In particular, it is the angle formed by two perpendiculars drawn to the same point on the line of intersection between the two planes.
The dihedral angle of two planes can be found from the definition of the dot product, 
, where 
and 
are the normal vectors to their respective planes.
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