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.
correct.
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.)
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.
You have correctly used Euler's formula for polyhedra, which states that in any polyhedron, the number of vertices (v), faces (f), and edges (e) are related by the equation v + f = e + 2.
To find the number of edges in a polyhedron with 13 vertices and 21 faces, you can rearrange the formula as e = v + f - 2. Plugging in the given values:
e = 13 + 21 - 2 = 34
Based on this calculation, the polyhedron has 34 edges, not 36.