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 July 1st, 2019, 02:49 PM #1 Senior Member   Joined: May 2015 From: Arlington, VA Posts: 458 Thanks: 29 Math Focus: Number theory Regular convex polygon in another What regular convex polygons of N sides, each equaling S, can fit into those of N-1 sides? Last edited by Loren; July 1st, 2019 at 02:57 PM.
 July 1st, 2019, 03:20 PM #2 Global Moderator   Joined: Dec 2006 Posts: 20,969 Thanks: 2218 None of them can.
 July 1st, 2019, 04:43 PM #3 Senior Member   Joined: May 2015 From: Arlington, VA Posts: 458 Thanks: 29 Math Focus: Number theory Then, does it make sense to ask: What regular convex polygons of N sides, each equaling S, can fit into those of N+1 sides?
July 1st, 2019, 04:51 PM   #4
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That does make sense, and my guess would be all of them.

As N increases, the angle defining one side gets narrower ($\displaystyle 2\pi/N rad$). If the side length remains constant, the circumscribed circle gets larger. Of course, we do have to check to make sure no vertices will poke out. It might be worth plotting up to N=10 or 20 to see what they look like.

(Edit: I got curious and plotted it. Not exactly a proof, but enough for me to put money on it.)
Attached Images
 Ngons.jpg (19.5 KB, 4 views)

Last edited by DarnItJimImAnEngineer; July 1st, 2019 at 05:19 PM.

July 1st, 2019, 07:14 PM   #5
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Math Focus: Number theory
Quote:
 Originally Posted by Loren Then, does it make sense to ask: What regular convex polygons of N sides, each equaling S, can fit into those of N+1 sides?
for all rotations about a common center?

 July 1st, 2019, 08:15 PM #6 Senior Member   Joined: Jun 2019 From: USA Posts: 196 Thanks: 78 Now that would be easier to (dis)prove. Find the expression for the radii of the inscribed and circumscribed circles (simple with trig functions) and see if $\displaystyle r_{outer}(N) < r_{inner}(N+1) \forall N >= 3$.
July 2nd, 2019, 02:29 PM   #7
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A regular triangle will not spin freely inside a square of the same side length. Nor will a square rotate in a pentagon. The pentagon and higher N-gons will.
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 Ngonspins.jpg (19.0 KB, 3 views)

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