# Diagonal – Definition with Examples

Updated on January 14, 2024

A diagonal is a line segment that can open up a world of exciting exploration in the realm of mathematics. It’s a straight line that streaks across polygons and polyhedra, connecting non-adjacent corners or vertices. Imagine a diagonal as a secret passageway within a shape that zips from one corner to a distant one, bypassing the adjacent corners. This may sound simple, but diagonals, like the mathematicians at Brighterly, love to delve into the intricacies of geometric structures. Be it a simple four-sided figure like a square or a quadrilateral or a more complex multi-sided polygon, diagonals help us unlock the mysteries of their structure and properties.

## What is a Diagonal?

A diagonal is a fascinating concept in the world of mathematics. By definition, it’s a straight line that connects two non-consecutive vertices of a polygon or polyhedron. In simpler terms, think of a diagonal as a shortcut across a shape, cutting from one corner to another without touching the corners immediately next to it. Whether it’s a simple quadrilateral or a more complex polygon, diagonals play a crucial role in understanding the structure and properties of these geometric figures.

## Diagonals in Geometry

In the context of geometry, diagonals serve as the building blocks of complex shapes. They are the lines that connect the vertices or corners of polygons and polyhedra. In a square, for instance, the diagonals not only bisect each other but are also equal in length, which is one of the distinctive properties of a square. In other polygons, the number and properties of diagonals can vary widely, providing a wealth of fascinating patterns and principles to explore.

## Diagonals of Polygons

### Diagonals of a Triangle

Technically, a triangle doesn’t have any diagonals. Why? Because diagonals connect non-adjacent vertices, and in a triangle, all vertices are adjacent to each other. So, no diagonals here!

### Diagonals of a Quadrilateral

In a quadrilateral, there are two diagonals. These diagonals might be equal (as in a square or rectangle) or different (as in a general quadrilateral). They bisect each other, forming four smaller triangles within the quadrilateral.

### Diagonals of a Square

In a square, the two diagonals are equal in length. They bisect each other at right angles, creating four identical right-angled triangles. Here’s an interesting tidbit: the length of the diagonal of a square is √2 times the side length of the square.

### Diagonals of a Rectangle

In a rectangle, the two diagonals are also equal in length, just like in a square. They bisect each other but do not necessarily intersect at right angles. They create four congruent triangles within the rectangle.

### Diagonals of a Rhombus

A rhombus has two diagonals that are not equal in length. However, they bisect each other at right angles. They divide the rhombus into four congruent triangles.

### Diagonals of a Pentagon

A pentagon, with five vertices, has ten diagonals. Each vertex of the pentagon is connected to the other three non-adjacent vertices by a diagonal.

### Diagonals of a Hexagon

A hexagon, with six vertices, has nine diagonals. Like a pentagon, each vertex of the hexagon is connected to the other four non-adjacent vertices by a diagonal.

## Diagonals of Solid Shapes

### Length of a Diagonal

The length of a diagonal can be calculated using the Pythagorean theorem in 2D shapes and the extension of this theorem in 3D shapes.

### Length of Diagonal of Square

The diagonal of a square can be calculated using the formula d = a√2, where ‘a’ is the length of the side of the square, and ‘d’ is the length of the diagonal.

### Length of Diagonal of a Rectangle

The diagonal of a rectangle can be calculated using the formula d = √(l² + w²), where ‘l’ is the length, ‘w’ is the width, and ‘d’ is the length of the diagonal.

### Length of a Diagonal of a Cube

The diagonal of a cube can be calculated using the formula d = a√3, where ‘a’ is the length of the side of the cube, and ‘d’ is the length of the diagonal.

### Length of a Diagonal of a Cuboid (Rectangular Prism)

The diagonal of a cuboid can be calculated using the formula d = √(l² + w² + h²), where ‘l’ is the length, ‘w’ is the width, ‘h’ is the height, and ‘d’ is the length of the diagonal.

## Diagonals of 3D Shapes

### Diagonals of a Cube

A cube, being a regular solid, has four space diagonals. All these diagonals are equal in length and pass through the center of the cube.

### Diagonals of a Cuboid

A cuboid has four space diagonals. These diagonals are not necessarily equal in length. They bisect each other and pass through the center of the cuboid.

## Practice Questions on Diagonals

To further your understanding of diagonals, try to answer these questions:

- How many diagonals does a decagon have?
- What about a dodecagon?
- Can you find a pattern to determine the number of diagonals in any polygon?

## Conclusion

Diagonals are more than just lines crisscrossing through shapes; they’re the secret keys that unlock the doors to understanding the complexities of geometry. They add layers of depth to the basic structure of polygons and polyhedra, revealing hidden patterns and properties that might otherwise remain unseen. Here at Brighterly, we believe in lighting up the path of learning with engaging and accessible content, and our exploration of diagonals is a perfect example of that mission. Whether you’re uncovering the properties of squares, rectangles, cubes, or diving deep into the world of complex shapes, diagonals serve as an exciting entry point into the fascinating world of mathematics. So, keep exploring, keep learning, and remember, every diagonal you draw takes you one step closer to becoming a master of shapes!

## Frequently Asked Questions on Diagonal

### What is a diagonal?

A diagonal is a line segment that connects two non-adjacent vertices in a polygon or polyhedron. In simpler terms, it’s like a shortcut that cuts across a shape from one corner to a non-adjacent one. Diagonals are integral to understanding the properties and structure of various geometric figures.

### How many diagonals does a polygon have?

The number of diagonals in a polygon depends on the number of its vertices or corners. For a polygon with ‘n’ vertices, the number of diagonals can be calculated using the formula n(n-3)/2. For instance, a pentagon (5-sided polygon) has 10 diagonals, while a hexagon (6-sided polygon) has 15 diagonals.

### How is the length of a diagonal calculated?

The length of a diagonal depends on the shape in question. In 2D shapes like squares and rectangles, we can use the Pythagorean theorem to calculate diagonal length. For 3D shapes like cubes and cuboids, we extend the Pythagorean theorem. For example, the diagonal of a rectangle can be found using the formula √(l² + w²), where ‘l’ is the length and ‘w’ is the width. For a cube, the diagonal is calculated as a√3, where ‘a’ is the side length.