Equilateral triangle

A triangle has three sides.

This one is very special. All three sides are the same length. All three corners are also the same.

This shape is called equilateral. It can make many patterns. You might see it on a road sign.

It can even make 3D shapes. Some shapes use only these triangles. It is a very neat shape.

An equilateral triangle is a special kind of triangle.

All three of its sides are the same length. All three of its corners, or angles, are also equal. Each angle measures 60 degrees. Because its sides and angles are equal, it is a regular polygon. This means it is a very balanced shape.

You can make this shape using a compass and a straightedge.

This shape is useful in many ways. You might see it on a yield sign on the road.

It also appears on the flags of Nicaragua and the Philippines. In science, it helps describe how some atoms connect in a flat plane.

These triangles can also build 3D shapes. Some shapes, called deltahedra, are made only of these triangles.

A regular octahedron is one such shape. These triangles can also fit together to cover a flat surface. They can make many different patterns or tilings. This shape is truly a building block for many things.

An equilateral triangle is a very special kind of shape. It is a triangle where all three sides have the exact same length. Because the sides are equal, the three corners, or angles, are also equal. Every angle in this triangle measures 60 degrees. This balance makes it a regular polygon. You might also hear it called a regular triangle. It is a special version of an isosceles triangle. An isosceles triangle only needs two equal sides, but the equilateral triangle has three.

There are many ways to study how this shape works. If you know the length of one side, you can find the perimeter easily. You just multiply the side length by three. You can also find the area using the height of the triangle. The height can be found using the Pythagoras theorem. If you put a point inside the triangle, a rule called Viviani's theorem applies. This rule says the sum of the distances from the point to the sides is always the same. This sum is equal to the triangle's altitude.

People have known how to make this shape for a very long time. The famous mathematician Euclid wrote about it in his book, the Elements. He showed how to draw one using only a compass and a straightedge. You can start with a line segment and use the compass to swing arcs. Where those arcs meet, you find the third corner. This method is one of the very first ideas in Euclid's work. It is a simple but perfect way to build the shape.

This shape appears in many different places in our world. You can see it on a yield sign while driving on the road. It is also found on the flags of Nicaragua and the Philippines. In science, it describes how some atoms connect in a flat plane. This is called trigonal planar molecular geometry. Even in math puzzles, it is important. For example, the triangle with the largest area for a set perimeter is always equilateral.

Equilateral triangles can also build amazing three-dimensional objects. Some shapes, called deltahedra, are made entirely of these triangles. A regular octahedron is one famous example of a deltahedron. These triangles can also fit together to cover a flat surface perfectly. This is called tiling or tessellation. They can even form complex fractal shapes like the Sierpiński triangle. From tiny atoms to huge buildings, this shape is everywhere.

An equilateral triangle is a unique geometric figure defined by perfect balance. It is a triangle where all three sides possess the exact same length. This equality of sides ensures that all three internal angles are also equal. Specifically, each angle in an equilateral triangle measures exactly 60 degrees. Because of these uniform properties, the shape is classified as a regular polygon. In modern geometry, it is considered a special case of an isosceles triangle. While an isosceles triangle requires at least two equal sides, the equilateral triangle satisfies this by having three.

To understand how this shape functions, we can look at its internal measurements. If you know the length of one side, the perimeter is simply three times that side length. Calculating the area requires knowing the height, or altitude, of the triangle. The altitude can be found using the Pythagoras theorem by looking at the difference between the square of a side and the square of half the base. Another way to find the area is through trigonometry, using the sine of a 60-degree angle. Mathematically, the equilateral triangle is quite efficient. A version of the isoperimetric inequality states that it is the triangle with the greatest area for any given perimeter.

This shape also has a very specific relationship with circles. You can draw a circumscribed circle that passes through all three vertices. You can also draw an inscribed circle that fits perfectly inside the triangle. The radius of the inscribed circle is exactly half the size of the circumradius. A theorem by Euler describes the distance between these two centers. This leads to a notable result: the equilateral triangle has the smallest ratio of circumradius to inradius of any triangle. Additionally, Pompeiu's theorem shows that any point in the plane can create a new triangle based on its distances to the vertices.

History shows us that humans have mastered this shape for centuries. The Greek mathematician Euclid included the construction of an equilateral triangle in his famous work, the Elements. He demonstrated how to create one using only a compass and a straightedge. One method involves drawing two intersecting circles of the same radius. Another method starts with a single line segment and uses arcs to find the third corner. This method remains a fundamental lesson in geometry. It proves that complex, perfect shapes can be built from very simple tools.

Beyond flat surfaces, equilateral triangles build complex three-dimensional structures. A polyhedron made entirely of equilateral triangles is called a deltahedron. There are eight strictly convex deltahedra in existence. Three of these are Platonic solids: the regular tetrahedron, the regular octahedron, and the regular icosahedron. The octahedron is a prominent example of this category.

Triangles also form the basis of antiprisms, which are polyhedra featuring a band of alternating triangles. This ability to form stable, repeating structures makes them vital in studying solid geometry.

We can see the influence of this shape in many practical areas. In architecture, the cross-section of the Gateway Arch features this geometry. In chemistry, the shape describes trigonal planar molecular geometry. This occurs when one central atom connects to three other atoms in a flat plane. You can also see the shape in daily life, such as on a yield sign used in traffic.

It even appears on the national flags of Nicaragua and the Philippines. These examples show how math moves from textbooks into the real world.

Finally, the equilateral triangle is a key component in many advanced mathematical patterns. It can tile the Euclidean plane perfectly, meaning they fit together without gaps. This process is known as tessellation. Some complex patterns, like the Sierpiński triangle, are fractals built by repeatedly dividing a large equilateral triangle into smaller ones. The shape also relates to the study of packing problems, such as fitting circles into the smallest possible triangle. Whether in the arrangement of atoms or the design of a city, the equilateral triangle remains a fundamental building block of our universe.