Diagonals
A diagonal is a line segment that joins one corner (vertex) of a polygon to another but is not an edge (side). In other words, it joins any two nonadjacent vertices of a polygon. So, we can draw the diagonals in a polygon when we directly join any two vertices which are not joined by any side. Let us learn more about the diagonal line, how to find the number of diagonals in a polygon, the diagonals of a triangle, if any, and the diagonals of a quadrilateral, along with some diagonal examples.
1.  What is a Diagonal? 
2.  Diagonals of Polygons 
3.  Number of Diagonals in a Polygon 
4.  Length of a Diagonal 
5.  FAQs on Diagonals 
What is a Diagonal?
A diagonal is a straight line that connects the opposite corners of a polygon through its vertices. In other words, the diagonal of a polygon is a line segment that joins any two nonadjacent corners. Different polygons may have different number of diagonals depending on the number of sides of the polygon.
Diagonal Shape
Since a diagonal is a line segment joining nonadjacent vertices or corners, the shape of a diagonal is that of a straight line.
Diagonals of Polygons
The diagonal of a polygon is a line segment that connects any two nonadjacent vertices. Depending upon the type of polygon and the number of sides, the number of diagonals and their properties vary.
Let us now learn more about the diagonals of different polygons.
Diagonals of a Triangle
A triangle is defined as a closed shape that has 3 sides, 3 angles, and 3 vertices. It is the simplest type of polygon. No vertices in a triangle are nonadjacent. It means that there are no line segments that can form diagonals. Therefore, the number of diagonals of a triangle = 0. In other words, there are no diagonals in a triangle.
Diagonals of a Quadrilateral
We know that a quadrilateral is a foursided polygon which means that if we join the two opposite vertices in it we get to form 2 diagonals and not more than that. In other words, there are 2 diagonals in a quadrilateral, whether it is a square, a rectangle, or a rhombus.
Diagonals of a Square
A square is defined as a closed twodimensional figure having four sides and four corners. The opposite sides are parallel to each other and all the sides are of equal length. The diagonal of a square is a line segment that joins any two of its opposite vertices. Observe the following square and its diagonals to relate the facts given below:
 The number of diagonals of a square = 2
 By joining the opposite vertices, we get two diagonals, AC and BD.
 The length of the diagonals AC and BD in the given square is the same.
 One diagonal of a square divides it into two equal right triangles, such that the diagonal becomes the hypotenuse of the rightangled triangles so formed.
Diagonals of a Rectangle
The diagonal of a rectangle is a line segment that joins any two of its nonadjacent vertices. Observe the following rectangle, in which AC and BD are the diagonals. We can see that AC and BD are of the same length. A diagonal divides a rectangle into 2 right triangles, and the diagonals become the hypotenuse of these rightangled triangles.
Diagonals of a Rhombus
The diagonals of a rhombus are the line segments joining the opposite vertices, bisecting each other at a 90° angle, which means that the two halves of any diagonal will be of the same length. A rhombus can be defined as a diamondshaped quadrilateral having four equal sides.
Diagonals of a Pentagon
A pentagon is a closed twodimensional figure with five sides and five corners. The length of all five sides is equal in a regular pentagon. A pentagon has 5 diagonals as shown in the figure given below.
Diagonals of a Hexagon
A hexagon is a closed twodimensional figure with six sides and six corners. The length of all six sides is equal in a regular hexagon. A hexagon has 9 diagonals as shown in the following figure.
Diagonals of 3D Shapes
Just like polygons, 3D shapes also have diagonals. Since they are threedimensional figures, they have body diagonals (space diagonals) and face diagonals. The number of diagonals and their properties vary for different solids, based on the number of edges. Let us learn more about the diagonal of a cube and the diagonal of a cuboid.
Diagonals of a Cube
A cube is a threedimensional solid figure, also known as a square solid that has edges of the same length. This means that the length, width, and height of a cube are equal, and all its faces are squares. The body diagonal of a cube is the line segment that cuts through its center, joining the opposite vertices. While the face diagonal is the one joining the opposite vertices on every face.
Diagonals of a Cuboid
A cuboid is a threedimensional form of a rectangle. The body diagonal of a cuboid is the one that cuts through the center of the cuboid; while the face diagonal is the one joining the opposite vertices on every face.
Number of Diagonals in a Polygon
The number of diagonals in a polygon differs according to the type of polygon, based on the number of sides. The formula that is used to find the number of diagonals in a polygon is,
Number of diagonals = n(n  3)/2
Here, 'n' represents the number of sides of the polygon. We can use this formula to find the number of diagonals of any polygon without actually drawing them.
The following table shows the number of diagonals of some polygons which is calculated using this formula.
Shape  Number of sides (n)  Number of Diagonals 

Triangle  3 
3(3  3)/2 = 0 
Quadrilateral  4 
4(4  3)/2 = 2 
Pentagon  5 
5(5  3)/2 = 5 
Hexagon  6 
6(6  3)/2 = 9 
Heptagon  7 
7(7  3)/2 = 14 
Octagon  8 
8(8  3)/2 = 20 
Nonagon  9 
9(9  3)/2 = 27 
Decagon  10 
10(10  3)/2 = 35 
Hendecagon  11 
11(11  3)/2 = 44 
Dodecagon  12 
12(12  3)/2 = 54 
Example: Find the number of diagonals in a decagon.
Solution:
The number of sides in a decagon is n = 10. The number of diagonals of a decagon is calculated using the formula:
n(n  3)/2 = 10(10  3)/2 = 10(7)/2 = 70/2 = 35
Therefore, the number of diagonals of a decagon = 35
Length of a Diagonal
The length of a diagonal for any polygon depends upon the type of polygon. There is no general formula to calculate the length of a diagonal. Rather, based on the dimensions of the particular polygon, the formula to calculate the length of the diagonal can be derived. The following section will cover the formula that is used to calculate the length of the diagonal for some polygons and solids based on their structure and dimensions.
Length of Diagonal of Square
The length of the diagonal of a square can be calculated using the Pythagoras theorem. Since the diagonal of a square divides the square into two rightangled triangles, the diagonal acts as a hypotenuse. So, applying the Pythagoras theorem, we can find the diagonal length using the formula, Diagonal (d) = √(x^{2 }+ x^{2}); where 'd' is the diagonal, and 'x' represents the side length. This formula further reduces to, d = √(x^{2 }+ x^{2}) = √(2x^{2}) = √2x
Length of diagonal of a square = √2x
Length of Diagonal of a Rectangle
The length of a diagonal of a rectangle can be calculated using the Pythagoras theorem because the diagonal of a rectangle divides the rectangle into two rightangled triangles. So, the diagonal acts as the hypotenuse, and the formula that is derived is, d^{2} = l^{2 }+ w^{2}. This further makes it, Length of a diagonal of a rectangle (d) = √(l^{2 }+ w^{2}); where 'l' represents the length and 'w' represents the width (breadth) of the rectangle.
Length of a Diagonal of a Cube
A cube has 6 faces and each face of a cube is a square. Thus, each face has two diagonals. Consider a cube of length x units. So, using the Pythagoras theorem, the length of each face diagonal of cube = √2x
Apart from the diagonals on the faces, there are 4 other diagonals (body diagonals) that pass through the center of the square. The formula for the length of the diagonal of a cube is derived in the same way as we derive the length of the diagonal of a square. Length of body diagonal of a cube = √3x
Length of a Diagonal of a Cuboid (Rectangular Prism)
Consider a cuboid of length l, width w, and height h. Let us assume that its body diagonal that passes through the center of the cuboid is 'd'. Length of the body diagonal of a cuboid = √(l^{2 }+ w^{2 }+ h^{2}).
Important Notes on Diagonals
 The number of diagonals of a polygon can be calculated with the formula, Number of diagonals = n(n  3)/2; where 'n' represents the number of sides in the polygon.
 The length of the diagonal of a rectangle with sides 'l' and 'w' is calculated with the formula, diagonal (d) = √(l^{2 }+ w^{2})
 The length of the diagonal of a cube is calculated with the formula, diagonal (d) = √3x; where 'x' is the side of a cube.
 The length of the diagonal of a cuboid of sides l, w, and h, is calculated with the formula, d = √(l^{2 }+ w^{2 }+ h^{2})
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Diagonal Examples

Example 1: If a polygon has 15 sides, how many diagonals does it have?
Solution:
Let us assume that the number of sides of the given polygon is 'n'. We can find the number of diagonals of the given polygon with the formula, Number of diagonals = n(n3)/2
Substituting the value of n = 15
n(n3)/2 = 15(153)/2 = (15 × 12)/2 = 90. Therefore, the polygon has 90 diagonals.

Example 2: If the length and width of a rectangle is 12 units and 5 units respectively. Find the length of its diagonal.
Solution:
Since the diagonal of a rectangle divides the rectangle into two rightangled triangles, the diagonal acts as a hypotenuse. So, applying the Pythagoras theorem, we can find the diagonal length using the formula, Diagonal (d) = √(l^{2 }+ w^{2}); where 'd' is the diagonal, 'l' is the length, and 'w' is the width of the rectangle. Given, l = 12 units, w = 5 units. Substituting the given values in the formula,
Diagonal (d) = √(l^{2 }+ w^{2})
d = √(12^{2 }+ 5^{2})
d = √(^{ }144 + 25)
d = √169 = 13 units. Therefore, the length of the diagonal = 13 units.

Example 3: State true or false:
a.) A triangle has only 1 diagonal.
b.) A quadrilateral has 4 diagonals.
Solution:
a.) False, a triangle has no diagonals.
b.) False, a quadrilateral has 2 diagonals.
FAQs on Diagonal
What is the Definition of Diagonal in Geometry?
The diagonal of a polygon is a line segment that joins any two nonadjacent vertices. In the case of a polygon, it is a straight line connecting the opposite corners of a polygon through its vertex. So, we get a diagonal when we directly join any two corners (vertices) which are not joined by an edge.
How to Find the Number of Diagonals in a Polygon?
The number of diagonals for any polygon differs according to the type of polygon, based on the number of sides. The formula to find the number of diagonals of a polygon is, Number of diagonals = n(n3)/2, where 'n' is the total number of sides of the polygon. For example, to find the number of diagonals in a quadrilateral, let us use the formula, Number of diagonals = n(n3)/2. Substituting the value of n = 4, we get, Number of diagonals = 4(43)/2 = (4 × 1)/2 = 2. So, we know that 2 diagonals can be drawn in a quadrilateral.
How many Diagonals does a Hexagon have?
A hexagon has 9 diagonals. This can be calculated with the help of the formula, Number of diagonals = n(n3)/2, where 'n' represents the number of sides. We know that a hexagon has 6 sides. On substituting the value of n = 6 in the formula we get, n(n3)/2 = 6(63)/2 = (6 × 3)/2 = 9.
How many Diagonals does an Octagon Hexagon have?
An octagon has 20 diagonals. This can be calculated with the help of the formula, number of diagonals = n(n3)/2, where 'n' represents the number of sides. We know that an octagon has 8 sides. On substituting the value of n = 8 in the formula we get, n(n3)/2 = 8(83)/2 = (8 × 5)/2 = 20.
How to Calculate the Length of Diagonal for any Shape?
The length of a diagonal for any polygon depends upon the type of polygon. There is no general formula to calculate the length of a diagonal. Rather, based on the dimensions of the particular polygon, the formula to calculate the length of the diagonal can be found. For example, the length of a diagonal of a square can be calculated with the formula, Diagonal length (d) = √2x; where 'x' is the side of the square.
How many Diagonals Does a Triangle Have?
A triangle has no diagonals. We know that a diagonal always connects any two nonadjacent (nonconsecutive) vertices of a polygon. Since all the vertices of a triangle are connected by sides, no diagonals can be formed.
How to Find the Diagonal of a Rectangle?
The length of the diagonal of a rectangle can be calculated if the length and width of the rectangle is given. Since the diagonal of a square divides the rectangle into rightangled triangles, the diagonal becomes the hypotenuse. So, using the Pythagoras theorem, the length of the diagonal can be found. For example, if 'd' is the diagonal, 'l' is the length, and 'w' is the width of the rectangle, then according to the Pythagoras theorem, d = √(l^{2 }+ w^{2})
What is the Formula for the Diagonal of a Square?
In order to find the diagonal of a square, we use the formula, d = √2x; where 'd' is the diagonal and 'x' is the side length. The formula is framed using the Pythagoras theorem. According to the Pythagoras theorem, d = √(x^{2 }+ x^{2}) = √(2x^{2}) = √2x, since we know that all the sides of a square are of equal length and the diagonal acts as a hypotenuse of the rightangled triangle that is formed by the diagonal.
What does a Diagonal Look Like?
The shape of a diagonal is a line segment. It starts and ends at the two opposite vertices of a polygon.
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