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Triangles. Properties and Pythagorean Theorem

14/04/2026

The online triangle simulations will help you to visualize graphically the characteristics and properties of triangles. We will see a visual proof of the Pythagorean theorem. We will learn what is the classification of triangles according to their sides and according to their angles and we will discover the many applications of triangles in real life.

This Thematic Unit is part of our Mathematics collection

STEM OnLine mini dictionary

Angle

The opening formed by two rays with a common origin called a vertex; its standard SI unit is the radian (rad), though it is commonly measured in degrees (°).

Cathetus

Each of the two sides that form the right angle in a right triangle.

Hypotenuse

The longest side in a right triangle, opposite the right angle.

Internal Angle

An angle formed by two adjacent sides of a polygon on its interior part.

Triangle

A polygon with three sides and three angles whose internal sum is always equal to 180°.

Triangle Height

The perpendicular segment drawn from a vertex to the opposite side or its extension.

Triangle Side

Each of the three line segments that bound the surface of the triangle.

Vertex

The joining point where two sides of a polygon meet and where the angle is formed.

What is a triangle?

Triangles are geometric figures with three sides and three angles. They are one of the most basic and fundamental shapes in Euclidean geometry. Triangles are classified according to their sides and angles.

Classification of triangles according to their sides: equilateral, isosceles and scalene triangles

In terms of sides, triangles can be equilateral, isosceles or scalene. An equilateral triangle has three equal sides, while an isosceles triangle has two equal sides and one different side. Finally, a scalene triangle has three different sides.

Classification of triangles according to their angles: acute, obtuse and right triangles

As for the angles, triangles can be acute, obtuse or right triangles. An acute triangle has all three acute angles, that is, less than 90 degrees. An obtuse triangle has one obtuse angle, that is, greater than 90 degrees. And a right triangle has a right angle of 90 degrees.

Properties of triangles

Triangles have unique properties and characteristics. The sum of the internal angles of a triangle is always equal to 180 degrees, which is one of the fundamental bases of geometry. In addition, any side of a triangle is always less than the sum of the other two sides, a property known as the triangular inequality, which ensures the viability of its shape. These properties are essential for understanding how triangles interact within more complex geometric figures, as well as in practical applications, such as the design of structures and mechanical systems where stability and precision are required.

Properties of right triangles. Pythagorean Theorem

The Pythagorean theorem is a fundamental rule in right triangles. It states that the square of the hypotenuse, which is the side opposite the 90 degree angle, equals the sum of the squares of the other two sides. Rectangular triangles are fundamental in trigonometry, where ratios such as sine, cosine and tangent are used to relate the sides and angles of these geometric figures.

Applications of triangles

In addition to their importance in Euclidean geometry, triangles are widely used in various areas, such as architecture, engineering and physics. Their simple and versatile structure makes them key elements in the construction of stable structures and in the description of natural and physical phenomena. In architecture, triangles are used to create geometric designs that distribute weight evenly and ensure the stability of buildings such as bridges and towers. In engineering, the properties of triangles are used to design mechanical and structural systems that support forces and movements. In physics, on the other hand, triangles are essential for modeling force dynamics, such as projectile trajectories and energy transmission. They also represent a fundamental component in the study of trigonometry, which is applied in fields as varied as navigation, astronomy and computational simulations.

STEM OnLine mini dictionary

Angle

The opening formed by two rays with a common origin called a vertex; its standard SI unit is the radian (rad), though it is commonly measured in degrees (°).

Cathetus

Each of the two sides that form the right angle in a right triangle.

Hypotenuse

The longest side in a right triangle, opposite the right angle.

Internal Angle

An angle formed by two adjacent sides of a polygon on its interior part.

Triangle

A polygon with three sides and three angles whose internal sum is always equal to 180°.

Triangle Height

The perpendicular segment drawn from a vertex to the opposite side or its extension.

Triangle Side

Each of the three line segments that bound the surface of the triangle.

Vertex

The joining point where two sides of a polygon meet and where the angle is formed.

Explore the exciting STEM world with our free, online, simulations and accompanying companion courses! With them you’ll be able to experience and learn hands-on. Take this opportunity to immerse yourself in virtual experiences while advancing your education – awaken your scientific curiosity and discover all that the STEM world has to offer!

Triangle simulations

Properties of right triangle


Visual proof of the Pythagorean Theorem


This triangle simulation is a visual proof of the Pythagorean Theorem. Move the pieces between squares and check that the area of the large square is equal to the sum of the areas of the small squares.

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Isaac Newton

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Test your knowledge

A triangle is a geometric figure made up of three sides and three angles, considered one of the basic structures of Euclidean geometry. Triangles can be classified according to their sides and their angles. By sides, they may be equilateral, with three equal sides; isosceles, with two equal sides and one different; or scalene, with all sides different. By angles, they may be acute, if all angles are less than 90 degrees; obtuse, if one angle is greater than 90 degrees; or right, if they contain a 90‑degree angle. These classifications help analyze their properties, solve geometric problems and understand how they relate to more complex figures.
Triangles have essential properties that support much of geometry. The most well‑known is that the sum of their interior angles is always 180 degrees, a key rule for solving geometric and trigonometric problems. Another fundamental property is the triangle inequality, which states that any side of a triangle is shorter than the sum of the other two. This ensures that the figure can close and exist physically. Triangles also serve as the basis for analyzing more complex structures, since their geometric rigidity makes them indispensable in architecture, engineering and design. Their stability allows the construction of bridges, towers and mechanisms requiring precision and strength.
Triangles matter because they combine simplicity and stability. In mathematics, they help study angles, lengths and relationships that later apply to more complex shapes. In real life, their rigidity makes them ideal for building safe structures: many bridges, roofs and towers use triangles because they do not deform easily. They also appear in navigation, physics and engineering, helping calculate distances, forces and trajectories. Even in digital design and simulations, triangles are used to build 3D models. They are small figures with a huge impact.
The Pythagorean theorem applies only to right triangles, those with a 90‑degree angle. It states that the square of the hypotenuse, the side opposite the right angle, equals the sum of the squares of the other two sides. This relationship allows unknown lengths to be calculated and is fundamental in trigonometry, physics, navigation and design.
Triangles are used in architecture, engineering, physics and many other disciplines because they are stable and easy to analyze. In construction, they distribute weight evenly and create strong structures such as bridges and towers. In engineering, they help design mechanisms that withstand forces and motion. In physics, they are used to study trajectories, break down forces and analyze systems. They are also essential in trigonometry, applied in navigation, astronomy and digital modeling.

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