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If a polyhedron has 13 vertices and 21 faces, how many edges does it have?

I just need to know if I am doing this right.
v+f=e+2
13+21=34+2=36 edges.

  • math -

    correct.

  • math -

    No

    v+f=e+2
    e=v+f-2=34-2=32

    For a simple polyhedron F - E + V = 2. (This is also called the Euler's Formula.)

  • math -

    Your equation is correct, but your answer is not. Your number of Edges is you e variable. so you are looking for the "e" what you found is "e+2"
    As a general Rule with math equations, and you can do this before you put your numbers in, figure out what variable you will be looking for (in this case e=?) and move everything else to the other side of your equals sign. This will let you have an otomatic solution, because it will set you equation up as "e=..." which is exactly what you want to know, and thn you don't have the confusion that you had in this set up again.
    so if your equation is v+f=e+2, e=?
    then your next step is to get e alone on one side.
    v+f=e+2
    -2= -2
    v+f-2=e
    then plug your numbers in to get your answer.

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