Posted by **marie** on Thursday, November 19, 2009 at 1:44pm.

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 -
**bobpursley**, Thursday, November 19, 2009 at 3:00pm
correct.

- math -
**bobpursley**, Thursday, November 19, 2009 at 3:03pm
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 -
**Shell**, Sunday, October 21, 2012 at 11:31am
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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