Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

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 $\psi$.

  1. Solve for the dihedral angle between by A and C.

  2. 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 $\psi$.

  3. Does this fall under trigonometry? If not, what?

share|cite|improve this question
You may want to tag this as differential geometry too. – user3180 Nov 29 '10 at 8:58
No, differential geometry involves derivatives. This is pure vector geometry. – Hans Lundmark Nov 29 '10 at 9:05

Question 1: The third angle is not uniquely determined by $\theta$ and $\psi$. If $a$, $b$ and $c$ are the unit normal vectors to the planes $A$, $B$ and $C$, then the problem is equivalent to determining $a \cdot c$ from $a \cdot b$ and $c \cdot b$, which cannot be done. (Think of $b$ being fixed; then giving values of $a \cdot b$ and $c \cdot b$ constrains $a$ and $c$ to lie on certain cones around $b$, but the positions of $a$ and $c$ on these cones are arbitrary.)

share|cite|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.