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

In a pyramid I have the side plans $25^{\circ}$ from vertical line. I will make a L-shaped support in the corners. In the CAD I can see it will be $100.29^{\circ}$ between the planes. How can calculate this with formulas? Could not attach picture in the system but I can seen separate.

share|cite|improve this question
I'm having a hard time parsing your question. Could you please upload your diagram to imgur? Someone will later embed the image into your question for you. – J. M. Sep 15 '11 at 12:19

Take $z$ axis up, and the base of the pyramid aligned with the $x$ and $y$ axes. I think you mean that one side plane has a normal vector $n = (0, \cos(25^\circ), \sin(25^\circ))$, and an adjacent side plane has normal $m = (\cos(25^\circ), 0,\sin(25^\circ))$.

Then the cosine of the angle between the perpendiculars is the dot product $$ n\cdot m = \sin^2(25^\circ) = 0.17861 $$ and this is equal to $(-\cos 100.29^\circ)$. The perpendiculars form an acute angle but your "L" fits inside, I assume, so is obtuse.

share|cite|improve this answer

Your Answer


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

Not the answer you're looking for? Browse other questions tagged or ask your own question.