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# Difference Between Diameter Diagonal

A circle is a geometric shape in which the locus of a point on a plane such that the distance of that point is constant from a fixed point on that same plane; this fixed point is called the centre of the circle, and the constant distance from the centre is called the radius of the circle. Similarly, a sphere is a three-dimensional closed figure whose surface is a set of points in a space equidistant to a fixed point, called the centre of the sphere. A polygon is a closed two-dimensional curve made of line segments. These line segments are called edges or sides of a polygon, and the common point of two edges is called vertices. For example, triangle, quadrilateral, pentagon etc. A polyhedron is a three-dimensional closed figure whose faces are a polygon. Some examples of polyhedrons are cube, cuboid, prism etc. Let us briefly discuss diameter and diagonal as a part of a circle, sphere, polygons and polyhedrons and differentiate between them.

## Diameter of a Circle

The chord of a circle is a line segment within the circle that joins any two points on the circle. And hence the longest chord of a circle that passes through the centre of the circle is called the diameter of the circle.

 Diameter is the most extended chord within a circle that passes through the centre of the circle. It is twice the radius of the circle, or half of the diameter is equal to the radius of the circle. It divides a circle into two equal parts. It is a line segment within a circle whose midpoint is the circle’s centre.

Now that we have seen what diameter is let us briefly discuss what is diagonal before making a differentiation between them.

## What is Diagonal?

For any closed curve made of line segments, whether 2-dimensional or 3-dimensional, a diagonal of a shape is a line segment that joins two vertices of the shape other than the edge of the shape.

Hence, a diagonal is a line segment whose endpoints are two non-adjacent vertices of the figure.

 Diagonal is a part of polygon or polyhedron and is a line segment that joins two non-adjacent vertices of the shape. The number of diagonals of a polygon with n vertices is given by the formula [n(n−3)]/2. Triangle is the only polygon that does not have a diagonal. Three-dimensional shapes have two types of diagonals – Body/Space Diagonals and Face Diagonals.

## Difference Between Diameter and Diagonal

 Diameter Diagonal It is a line segment within a circle joining any two points on the circle and passing through its centre. It is part of a circle, sphere or hemisphere. There can be an infinite number of diameters drawn within a circle. It may be a slanted or straight-line segment. It divides a circle or a sphere into two equal parts. For any circle, the diameter is twice the radius of the circle.  It necessarily lies in the interior of the circle or sphere. It is a line segment that joins any non-adjacent vertices of a polygon or a polyhedron. It is a part of a polygon or a polyhedron. A polygon or a polyhedron has a finite number of diagonals. It is a necessarily slanted or oblique line segment. A diagonal does not always divide the shape into equal parts. The diagonal length depends on the length of the edges of a given regular polygon or polyhedron. It may lie on the exterior of the shape. For example, in the case of a concave polygon, its diameter lies in the exterior.

Hence, diameter is related to a circle, and all its geometrical properties are related to the properties of a circle. In contrast, a diagonal is associated with a polygon. Despite both being a line segment joining any two points on a geometric figure, they are different.

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