Eulerβs formula for Polyhedra gives the basic condition for any three-dimensional shape being polyhedra. Polyhedra, plural of a polyhedron, is a three-dimensional closed figure whose faces are flat and polygonal, edges are made of straight lines and corners are sharp. Example of polyhedrons is pyramids, tetrahedrons, etc.
In 1758, a Swiss mathematician Leonhard Euler gave a formula for convex polyhedrons relating their faces, edges and vertices. This became the condition for any three-dimensional figure being a convex polyhedron.
Learn more about polyhedrons – here.
What is the Eulerβs Formula for Polyhedrons?
Despite the straightforward definition of polyhedra, it was quite difficult to categorise them explicitly. Eulerβs characteristic equation gave an important condition for the surfaces of polyhedrons. The characteristic equation is given as
Ο = V β E + F, where
V is the number of vertices of the polyhedra,
E is the number of edges, and
F is the number of faces of polyhedra.
For convex polyhedrons, Ο = 2. Thus we have Eulerβs formula for polyhedrons as
|
Let us check this formula for the five Platonic solids:
S.No. |
Polyhedron Name |
Shape |
Vertices V |
Edges E |
Faces F |
Eulerβs Formula V β E + F = 2 |
1. |
Tetrahedron |
|
4 |
6 |
4 |
4 β 6 + 4 = β2 + 4 = 2 |
2. |
Hexahedron or Cube |
|
8 |
12 |
6 |
8 β 12 + 6 = β 4 + 6 = 2 |
3. |
Octahedron |
|
6 |
12 |
8 |
6 β 12 + 8 = β 6 + 8 = 2 |
4. |
Icosahedron |
|
12 |
30 |
20 |
12 β 30 + 20 = β18 + 20 = 2 |
5. |
Dodecahedron |
|
20 |
30 |
12 |
20 β 30 + 12 = β 10 + 12 = 2. |
Proof of Eulerβs Formula
We shall prove Eulerβs formula using graph theory.
Refer
- Graph theory in discrete mathematics
- Types of Graphs
We prove the formula by applying induction on edges by considering the polyhedra as a simply connected planar graph G with v vertices, e edges and f faces.
If G has zero number of edges, that is e = 0. The graph will have a single with f = 1. Then, by the Eulerβs formula V β E + F = 1 β 0 + 1 = 2. Thus, it satisfies Eulerβs formula.
Induction step: Let the Eulerβs formula is applicable for a graph with n edges.
Let G be a graph with n + 1 edges.
Case I: If G is a tree and does not contain any cycle. It can be easily proven by induction for trees with any number of edges.
Case II: If G is not a tree and contains atleast one cycle. Choose an edge e1 in G which divides the given region into two different parts and remove that edge e1 to get another graph Gββ.
Faces in G > Faces in Gββ
Now, G has n + 1 edges, then Gββ has n edges so by the hypothesis Gββ satisfies the Eulerβs formula. For Gββ, Vβ β Eβ + Fβ = 2 where Vβ = v, Eβ = e β 1 and Fβ = f β 1.
Now, substituting these values, we get
Vβ β Eβ + Fβ = 2
β v β e + 1 + f β 1 = 2
β v β e + f = 2
Hence, Eulerβs formula is applicable for n + 1 edges.
This proves the Eulerβs formula.
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Solved Examples on Eulerβs Formula for Polyhedra
Example 1:
Prove the Eulerβs formula for a triangular prism.
Solution:
Number of vertices of a triangular prism = 6
Number of edges of a triangular prism = 9
Number of faces of a triangular prism = 5
By the Eulerβs formula
V β F + E = 6 β 9 + 5 = β 3 + 5 = 2
β V β F + E = 2
Thus, Eulerβs formula is satisfied for a triangular prism.
Example 2:
Is cuboid a polyhedron?
Solution:
If cuboid is a polyhedron then it must satisfy Eulerβs formula for polyhedra.
Number of edges, e = 12
Number of vertices, v = 8
Number of faces, f = 6
By the Eulerβs formula:
v β e + f = 8 β 12 + 6 = β4 + 6 = 2.
β v β e + f = 2.
which satisfies the Eulerβs formula. Hence, the cuboid is a polyhedron.
Practice Questions:
1. Is it possible to have a polyhedron with three triangular faces?
2. Complete the following table:
S.No. |
Faces |
Edges |
Vertices |
1. |
6 |
12 |
β¦ |
2. |
β¦ |
30 |
12 |
3. |
12 |
β¦ |
20 |
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Frequently Asked Questions on Eulerβs Formula
What is Eulerβs formula for solid shapes?
Eulerβs characteristic formula for polyhedrons is Ο = V β E + F and for convex polyhedrons Ο = 2.
What is the formula for polyhedron?
Eulerβs formula for polyhedra is V β E + F = 2 where V is the number of vertices, E is the number of edges and F is the number of faces of a polyhedron.
Does Eulerβs formula work for all polyhedra?
There are few constraints for the applicability of Eulerβs formula:
- The polyhedron must not have holes in it.
- It must not intersect itself.
- It must not be made by connecting two polygons.
What is the Eulerβs formula for a polyhedra for class 8?
For a polyhedron, the Eulerβs formula is given by V β E + F = 2 where V is the number of vertices, E is the number edges and F is the number of faces of a polyhedron.
Is a cylinder a polyhedron?
No, a cylinder is not a polyhedron.
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