# Pyramid and angle between planes

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.

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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.