by Alexander on October 28th, 2009

Alexander

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It is given that a polyhedron has 12 faces and 20 vertices. How many edges are there?

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  • by iwnit on October 28th, 2009

    iwnit

    1) Assuming a convex polyhedron, Euler's formula would applies:
    V-E+F = 2
    where
    V= 20 [number of vertexes]
    E= unknown [number of edges]
    F=12 [number of faces]

    After substitution:
    20-E+12=2
    E= 30
    There would be 30 edges.

    An example would be a regular dodecahedron.

    Further information:
    http://en.wikipedia.org/wiki/Dodecahedron
    http://home.att.net/~numericana/data/polyhedra.htm


    2) "The Euler characteristic χ was classically defined for the surfaces of polyhedra, according to the formula
    χ = V-E+F
    where V, E, and F are respectively the numbers of vertices (corners), edges and faces in the given polyhedron. Any convex polyhedron's surface has Euler characteristic
    χ = V-E+F = 2
    This result is known as Euler's formula."

    "The surfaces of nonconvex polyhedra can have various Euler characteristics"
    Source and further information:
    http://en.wikipedia.org/wiki/Euler's_polyhedron_formula

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