Distance between centers on a dodecahedron surface

Geometry Level pending

Given a regular dodecahedron of unit edge length, find the shortest distance on the surface of the dodecahedron between the centers of two opposite faces. If the distance is d d then find 1000 d \lfloor 1000 d \rfloor . The path between the two centers has to be on the surface of the dodecahedron.

Hint: Unfold (unwrap) the faces of the dodecahedron, and connect the two centers with a straight line.


The answer is 3950.

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1 solution

Hosam Hajjir
Jun 13, 2020

The following animation depicts unfolding of the faces of the dodecahedron. The last frame shows how to compute the distance between the two dots using two congruent triangles with two of the sides being a a and 2 a 2 a , where a a is the apothem of the pentagon, a = 1 2 cot π 5 a =\frac{1}{2} \cot \frac{\pi}{5} . And it follows that,

d = 2 4 a 2 + a 2 4 a 2 cos ( 4 π 5 ) = 3.950016598 d = 2 \sqrt{ 4 a^2 + a^2 - 4 a^2 \cos( \frac{4 \pi}{5} ) } = 3.950016598 . Hence the answer is 3950 \boxed{3950} .

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