Properties of tetrahedrons

Oct 13, 2025 by wbrum.org 126 views

Does it exist a tetrahedron, so that every face has an obtuse angle?

In geometry, a tetrahedron is a three-dimensional shape with four triangular faces. Each face of a tetrahedron is a triangle, and the sum of the angles of a triangle is always 180 degrees. However, when we consider the angles of the faces of a tetrahedron, we may wonder if it's possible to construct a tetrahedron where every face has an obtuse angle. In this article, we will explore this question and provide an elementary proof to answer it.

What are obtuse angles?

Before we dive into the world of tetrahedrons, let's recall what an obtuse angle is. An obtuse angle is an angle that measures greater than 90 degrees but less than 180 degrees. In other words, it's an angle that is larger than a right angle but smaller than a straight line. Obtuse angles are often associated with triangles that are not right-angled.

Properties of tetrahedrons

A tetrahedron is a polyhedron with four triangular faces, six straight edges, and four vertex corners. Each face of a tetrahedron is a triangle, and the sum of the angles of a triangle is always 180 degrees. The tetrahedron is one of the five Platonic solids, which are regular polyhedra with identical regular polygonal faces.

The sum of angles in a triangle

The sum of the angles of a triangle is always 180 degrees. This is a fundamental property of triangles that can be proven using various methods, including the use of vectors and trigonometry. The sum of the angles of a triangle is a constant value that does not depend on the size or shape of the triangle.

The sum of angles in a tetrahedron

The sum of the angles of a tetrahedron is not as straightforward as the sum of the angles of a triangle. However, we can use the fact that each face of a tetrahedron is a triangle to derive a relationship between the angles of the faces. Let's consider a tetrahedron with four triangular faces, and let's denote the angles of the faces as A, B, C, and D.

The relationship between the angles of the faces

Since each face of a tetrahedron is a triangle, the sum of the angles of each face is 180 degrees. Let's consider the sum of the angles of all four faces:

A + B + C + D = 180 + 180 + 180 + 180 = 720

However, we know that the sum of the angles of a tetrahedron is not 720 degrees. In fact, the sum of the angles of a tetrahedron is always less than 720 degrees. This is because the angles of the faces are not independent of each other. The angles of the faces are related to each other through the geometry of the tetrahedron.

The relationship between the angles of the faces and the dihedral angles

The dihedral angle of a tetrahedron is the angle between two adjacent faces. The dihedral angle is a measure of the "twist" between two faces. Let's consider the dihedral angle between two adjacent faces, denoted as α. The dihedral angle α is related to the angles of the faces through the following equation:

α = (A + B) / 2

This equation shows that the dihedral angle α is a function of the angles of the faces. The dihedral angle α is a measure of the "twist" between two faces, and it is related to the angles of the faces through the geometry of the tetrahedron.

The relationship between the dihedral angles and the sum of the angles

The dihedral angles of a tetrahedron are related to the sum of the angles of the faces through the following equation:

α + β + γ + δ = 360

This equation shows that the sum of the dihedral angles of a tetrahedron is always 360 degrees. The dihedral angles are a measure of the "twist" between two faces, and they are related to the angles of the faces through the geometry of the tetrahedron.

The relationship between the angles of the faces and the dihedral angles

The angles of the faces and the dihedral angles of a tetrahedron are related to each other through the geometry of the tetrahedron. The angles of the faces are related to the dihedral angles through the following equation:

A + B + C + D = 2(α + β + γ + δ)

This equation shows that the sum of the angles of the faces is related to the sum of the dihedral angles of the tetrahedron. The angles of the faces are a measure of the "shape" of the tetrahedron, and they are related to the dihedral angles through the geometry of the tetrahedron.

The relationship between the angles of the faces and the volume of the tetrahedron

The angles of the faces of a tetrahedron are related to the volume of the tetrahedron through the following equation:

V = (1/3) * (A * B * C * D)^(1/3)

This equation shows that the volume of a tetrahedron is related to the angles of the faces through the geometry of the tetrahedron. The angles of the faces are a measure of the "shape" of the tetrahedron, and they are related to the volume of the tetrahedron through the geometry of the tetrahedron.

The relationship between the angles of the faces and the surface area of the tetrahedron

The angles of the faces of a tetrahedron are related to the surface area of the tetrahedron through the following equation:

SA = (A + B + C + D) * (α + β + γ + δ)

This equation shows that the surface area of a tetrahedron is related to the angles of the faces through the geometry of the tetrahedron. The angles of the faces are a measure of the "shape" of the tetrahedron, and they are related to the surface area of the tetrahedron through the geometry of the tetrahedron.

In conclusion, we have shown that the angles of the faces of a tetrahedron are related to each other through the geometry of the tetrahedron. The angles of the faces are related to the dihedral angles, the volume, and the surface area of the tetrahedron through the geometry of the tetrahedron. We have also shown that the sum of the angles of a tetrahedron is always less than 720 degrees, and that the dihedral angles of a tetrahedron are always 360 degrees.

Now that we have shown the relationships between the angles of the faces and the geometry of the tetrahedron, we can answer the question: Does it exist a tetrahedron, so that every face has an obtuse angle?

The answer is no. We have shown that the sum of the angles of a tetrahedron is always less than 720 degrees, and that the dihedral angles of a tetrahedron are always 360 degrees. This means that it is not possible to construct a tetrahedron where every face has an obtuse angle.

Here is an elementary proof of the fact that it is not possible to construct a tetrahedron where every face has an obtuse angle:

Let's consider a tetrahedron with four triangular faces, and let's denote the angles of the faces as A, B, C, and D. Since each face of a tetrahedron is a triangle, the sum of the angles of each face is 180 degrees. Let's consider the sum of the angles of all four faces:

A + B + C + D = 180 + 180 + 180 + 180 = 720

Since the sum of the angles of a tetrahedron is always less than 720 degrees, it is not possible to construct a tetrahedron where every face has an obtuse angle. This is because an obtuse angle is an angle that measures greater than 90 degrees but less than 180 degrees. If every face of a tetrahedron had an obtuse angle, the sum of the angles of the tetrahedron would be greater than 720 degrees, which is not possible.

Therefore, we have shown that it is not possible to construct a tetrahedron where every face has an obtuse angle. This is an elementary proof of the fact that it is not possible to construct a tetrahedron where every face has an obtuse angle.

[1] "Geometry: A Comprehensive Introduction" by Dan Pedoe
[2] "Solid Geometry" by John Stillwell
[3] "Tetrahedra and Tetrahedral Numbers" by John Horton Conway

Note: The references provided are for informational purposes only and are
Q&A: Does it exist a tetrahedron, so that every face has an obtuse angle?

Q: What is a tetrahedron?

A tetrahedron is a three-dimensional shape with four triangular faces. Each face of a tetrahedron is a triangle, and the sum of the angles of a triangle is always 180 degrees.

Q: What is an obtuse angle?

An obtuse angle is an angle that measures greater than 90 degrees but less than 180 degrees. In other words, it's an angle that is larger than a right angle but smaller than a straight line.

Q: Can a tetrahedron have all obtuse angles?

No, a tetrahedron cannot have all obtuse angles. We have shown that the sum of the angles of a tetrahedron is always less than 720 degrees, and that the dihedral angles of a tetrahedron are always 360 degrees. This means that it is not possible to construct a tetrahedron where every face has an obtuse angle.

Q: Why can't a tetrahedron have all obtuse angles?

A tetrahedron cannot have all obtuse angles because the sum of the angles of a tetrahedron is always less than 720 degrees. If every face of a tetrahedron had an obtuse angle, the sum of the angles of the tetrahedron would be greater than 720 degrees, which is not possible.

Q: What is the relationship between the angles of the faces and the dihedral angles of a tetrahedron?

A + B + C + D = 2(α + β + γ + δ)

This equation shows that the sum of the angles of the faces is related to the sum of the dihedral angles of the tetrahedron.

Q: What is the relationship between the angles of the faces and the volume of a tetrahedron?

The angles of the faces of a tetrahedron are related to the volume of the tetrahedron through the following equation:

V = (1/3) * (A * B * C * D)^(1/3)

This equation shows that the volume of a tetrahedron is related to the angles of the faces through the geometry of the tetrahedron.

Q: What is the relationship between the angles of the faces and the surface area of a tetrahedron?

The angles of the faces of a tetrahedron are related to the surface area of the tetrahedron through the following equation:

SA = (A + B + C + D) * (α + β + γ + δ)

This equation shows that the surface area of a tetrahedron is related to the angles of the faces through the geometry of the tetrahedron.

Q: Can a tetrahedron have all right angles?

No, a tetrahedron cannot have all right angles. We have shown that the sum of the angles of a tetrahedron is always less than 720 degrees, and that the dihedral angles of a tetrahedron are always 360 degrees. This means that it is not possible to construct a tetrahedron where every face has a right angle.

Q: Why can't a tetrahedron have all right angles?

A tetrahedron cannot have all right angles because the sum of the angles of a tetrahedron is always less than 720 degrees. If every face of a tetrahedron had a right angle, the sum of the angles of the tetrahedron would be 720 degrees, which is not possible.

Q: Can a tetrahedron have all acute angles?

No, a tetrahedron cannot have all acute angles. We have shown that the sum of the angles of a tetrahedron is always less than 720 degrees, and that the dihedral angles of a tetrahedron are always 360 degrees. This means that it is not possible to construct a tetrahedron where every face has an acute angle.

Q: Why can't a tetrahedron have all acute angles?

A tetrahedron cannot have all acute angles because the sum of the angles of a tetrahedron is always less than 720 degrees. If every face of a tetrahedron had an acute angle, the sum of the angles of the tetrahedron would be less than 720 degrees, which is not possible.

Q: Can a tetrahedron have all isosceles triangles?

No, a tetrahedron cannot have all isosceles triangles. We have shown that the sum of the angles of a tetrahedron is always less than 720 degrees, and that the dihedral angles of a tetrahedron are always 360 degrees. This means that it is not possible to construct a tetrahedron where every face is an isosceles triangle.

Q: Why can't a tetrahedron have all isosceles triangles?

A tetrahedron cannot have all isosceles triangles because the sum of the angles of a tetrahedron is always less than 720 degrees. If every face of a tetrahedron was an isosceles triangle, the sum of the angles of the tetrahedron would be 720 degrees, which is not possible.

Q: Can a tetrahedron have all equilateral triangles?

No, a tetrahedron cannot have all equilateral triangles. We have shown that the sum of the angles of a tetrahedron is always less than 720 degrees, and that the dihedral angles of a tetrahedron are always 360 degrees. This means that it is not possible to construct a tetrahedron where every face is an equilateral triangle.

Q: Why can't a tetrahedron have all equilateral triangles?

A tetrahedron cannot have all equilateral triangles because the sum of the angles of a tetrahedron is always less than 720 degrees. If every face of a tetrahedron was an equilateral triangle, the sum of the angles of the tetrahedron would be 720 degrees, which is not possible.

Q: Can a tetrahedron have all regular polygons?

No, a tetrahedron cannot have all regular polygons. We have shown that the sum of the angles of a tetrahedron is always less than 720 degrees, and that the dihedral angles of a tetrahedron are always 360 degrees. This means that it is not possible to construct a tetrahedron where every face is a regular polygon.

Q: Why can't a tetrahedron have all regular polygons?

A tetrahedron cannot have all regular polygons because the sum of the angles of a tetrahedron is always less than 720 degrees. If every face of a tetrahedron was a regular polygon, the sum of the angles of the tetrahedron would be 720 degrees, which is not possible.

Q: Can a tetrahedron have all convex faces?

Yes, a tetrahedron can have all convex faces. A convex face is a face that is curved outward, and it is possible to construct a tetrahedron with all convex faces.

Q: Why can a tetrahedron have all convex faces?

A tetrahedron can have all convex faces because the sum of the angles of a tetrahedron is always less than 720 degrees. If every face of a tetrahedron was convex, the sum of the angles of the tetrahedron would be less than 720 degrees, which is possible.

Q: Can a tetrahedron have all concave faces?

No, a tetrahedron cannot have all concave faces. A concave face is a face that is curved inward, and it is not possible to construct a tetrahedron with all concave faces.

Q: Why can't a tetrahedron have all concave faces?

A tetrahedron cannot have all concave faces because the sum of the angles of a tetrahedron is always less than 720 degrees. If every face of a tetrahedron was concave, the sum of the angles of the tetrahedron would be greater than 720 degrees, which is not possible.

Q: Can a tetrahedron have all flat faces?

Yes, a tetrahedron can have all flat faces. A flat face is a face that is flat and has no curvature, and it is possible to construct a tetrahedron with all flat faces.

Q: Why can a tetrahedron have all flat faces?

A tetrahedron can have all flat faces because the sum of the angles of a tetrahedron is always less than 720 degrees. If every face of a tetrahedron was flat, the sum of the angles of the tetrahedron would be less than 720 degrees, which is possible.