Three planes A, B and C intersect at point P. The dihedral angle between A and B is $\theta$ and the dihedral angle between B and C is $\psi$.
Solve for the dihedral angle between by A and C.
Planes A and B intersect plane C in two lines that form angle QPR. That is, point Q lies in both A and C, and point R lies in both B and C. Solve for the angle of QPR in terms of $\theta$ and $\psi$.
Does this fall under trigonometry? If not, what?