Diagonals of Parallelogram: Formula, Examples

As we journey through the intriguing world of geometry, we come across a multitude of shapes, each holding its own secrets and marvels. One such shape is the parallelogram, a fascinating quadrilateral that has captured the curiosity of mathematicians throughout history. At Brighterly, we believe in kindling the innate curiosity within every child, leading them to explore the realms of mathematics in a fun, engaging, and enlightening manner.

In this context, we invite our young learners to delve into the concept of diagonals in a parallelogram. A diagonal is a line segment that connects two non-adjacent vertices of a polygon. Diagonals hold a special place in geometry, linking various properties of shapes together and illuminating the inherent symmetry and elegance within them. For parallelograms, understanding the properties of diagonals opens a gateway to a deeper appreciation of the figure’s structure, properties, and relations with other shapes. Let’s embark on this exciting journey, unleashing the hidden secrets of parallelograms and their diagonals.

What Are Diagonals of a Parallelogram?

Imagine a parallelogram, a four-sided figure with opposite sides of equal length, much like a slanted rectangle. Now imagine a line connecting one corner of this parallelogram to the opposite corner. This line is known as the diagonal. Diagonals in a parallelogram are unique because they have several distinctive properties that set them apart from diagonals in other types of quadrilaterals. As such, diagonals play an instrumental role in a variety of mathematical theorems and proofs, particularly within the fascinating field of geometry. Children who aspire to excel in mathematics will find it rewarding to delve into the intriguing world of parallelogram diagonals, thereby increasing their understanding of geometrical concepts and their applications in real-world scenarios.

Properties of Diagonal of Parallelogram

The properties of the diagonals in a parallelogram are intriguing and unique. One key property is that these diagonals bisect each other, which means they divide each other into two equal parts at the point of intersection. Secondly, while the diagonals are not necessarily equal in length, they do cut the parallelogram into two congruent triangles. This property is critical in proving various theorems related to parallelograms and triangles. Furthermore, it’s also worth noting that the diagonals in a rectangle or a square (both special types of parallelograms) are equal in length, making these figures even more unique in the wonderful world of geometry.

Diagonal of Parallelogram Formula

The formula to find the length of diagonals in a parallelogram is based on the lengths of the sides and the angle between them. For a parallelogram with sides of lengths a and b and an included angle θ, the diagonals p and q are given by the formulas:

p = √(a² + b² + 2ab cos θ)
q = √(a² + b² - 2ab cos θ)

These formulas are derived using the Law of Cosines, a fundamental principle in trigonometry, and allow us to compute the lengths of the diagonals given the sides and the included angle.

Finding Lengths of Diagonals of a Parallelogram

To find the lengths of the diagonals of a parallelogram, you need to know the lengths of the sides and the measure of the included angle. Then, you can plug these values into the formulas mentioned above. This calculation may initially seem complex, but with practice, it becomes a straightforward application of the formulas. The lengths of the diagonals provide valuable information about the shape and properties of the parallelogram, which can be used to solve various mathematical problems.

Relationship Between Sides and Diagonals of a Parallelogram

There is an interesting relationship between the sides and diagonals of a parallelogram. As mentioned earlier, the diagonals bisect each other, forming four triangles within the parallelogram. Each of these triangles shares a side with the parallelogram and has a diagonal as its hypotenuse. This relationship is instrumental in deriving the formulas for the lengths of the diagonals. Moreover, it illustrates how the sides and diagonals of a parallelogram are interconnected, reflecting the inherent harmony within geometrical figures.

Solved Examples on Diagonals of Parallelogram

Let’s look at some solved examples to understand these concepts better. Suppose we have a parallelogram with sides of lengths 5 units and 7 units, and an included angle of 60 degrees. Plugging these values into our formulas, we find that the lengths of the diagonals are approximately 8.66 units and 7.21 units. These examples help illuminate the application of the formulas and their use in determining the properties of a parallelogram.

Practice Problems on Diagonals of Parallelogram

Try out these practice problems to test your understanding:

  1. Find the lengths of the diagonals of a parallelogram with side lengths of 4 units and 6 units, and an included angle of 45 degrees.
  2. Determine the side lengths of a parallelogram if the lengths of the diagonals are 10 units and 8 units, and the included angle is 90 degrees.

Working on these problems will enhance your understanding and provide practical experience with the formulas and concepts discussed.


Decoding the mystery of diagonals in a parallelogram equips young learners with the tools to navigate the fascinating world of geometry with confidence and ease. At Brighterly, we believe that learning is a joyful exploration that lights the path to a world of possibilities. Understanding these mathematical concepts not only strengthens the foundational knowledge but also fosters analytical thinking, problem-solving skills, and a lifelong love for learning.

Diagonals might just be lines drawn across a shape, but they carry within them the beauty and intricacy of the mathematical world. By understanding the properties, formulas, and applications associated with these diagonals, you take a step closer to mastering the concepts that underlie geometry. Let the journey of exploration continue, as we uncover more geometric secrets together!

Frequently Asked Questions on Diagonal of Parallelogram

Why are the diagonals of a parallelogram important?

The diagonals of a parallelogram hold many interesting properties, such as bisecting each other, and in certain types of parallelograms, being equal in length. These properties are often used in proofs and problem-solving in geometry, making the understanding of diagonals a key part of a solid mathematical foundation.

What happens to the diagonals in a special parallelogram, like a rectangle?

In a rectangle or a square (both special types of parallelograms), the diagonals are not only equal in length, but they also bisect each other at right angles. This adds an additional layer of symmetry to these shapes and results in more fascinating properties to explore!

How does the length of the diagonals relate to the sides of a parallelogram?

The length of the diagonals in a parallelogram is determined by the lengths of the sides and the angle between them. This relationship can be expressed by specific formulas derived from the Law of Cosines, a fundamental principle in trigonometry. Understanding this relationship can offer deep insights into the geometric properties of a parallelogram.


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