Edge-Ploding Polyhedra

Geometry Level 2

Let P P be a polyhedron that is the convex hull of the vertices v 1 , v 2 , , v n v_1, v_2, \ldots , v_n and with edges e 1 , e 2 , , e m e_1, e_2, \ldots , e_m where each edge is defined to be the set of points on the line that passes through some two distinct vertices v i , v j v_i, v_j .

We define the Edge-Plosion of P P , E ! ( P ) E!( P ) , to be the convex hull of the mid-points of each edge of P P , where P P is a cube.

Which of these is an image of E ! ( E ! ( P ) ) E!(E!(P)) ?

(A) :

(B) :

(C) :

(D) :

A B C D

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1 solution

Michael Mendrin
Aug 12, 2016

We can look at this as whittling a cube block of wood. No matter how many "edge explosions" is done to the cube, there will always be 6 6 square faces centered with the original 6 6 faces of the cube. Only A A and C C has that property, the others are eliminated.

Hi Micheal,

I have a programme to "create" these objects in Python and rotate them around. Using this its clear that A also has this property but its not very clear from the perspective that was in the original image ( It's always a pain trying to visualise 3d objects in still images effectively!! ). I've changed this now.

Roberto Nicolaides - 4 years, 10 months ago

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That would be after the first "edge explosion", yes. C would be the next. I've fixed my first reply.

Michael Mendrin - 4 years, 10 months ago

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Yes, exactly :)

Roberto Nicolaides - 4 years, 10 months ago

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