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Apolloniu's Theorem

The world of mathematics is a field that has many theorems, and each gives or provides a unique perspective on the complexity of geometric shapes; one such important Theorem is "Apollonius' Theorem." It is a result that shows the relationship between sides of triangles; this Theorem is named after the ancient Greek mathematics Apollonius of Perga. This Theorem has been a source of fascination and usefulness for mathematics and students alike for centuries. In this article, we will get to know about the concept of Apollonius' Theorem in a detailed manner, understanding its history, geometric importance, and applications, and end this article with a conclusion.


To get a better understanding of the importance of Apollonius' Theorem, we have first to get that this Theorem is named after Apollonius of Parge. He was born in the early 3rd century BCE; he was a famous Greek mathematician known for his contribution to the field of geometry and conic section. His work is used as the base for future development in the field of mathematics and science.

Apollonius' Theorem is a result that gives the relation between the median and sides of a triangle. It was first presented in his monumental work "Conics," a detailed book on the properties and classifications of conic sections; the Theorem appears in Book I, Proposition 6 of "Conics," where Apollonius explores the relationships between circles and triangles.

Definition of Apollonius' Theorem

In a triangle, "if we square the length of two sides and add them together, the result is equal to the sum of the square of half the length of the third side and the square of the length of the median of the third side"; this Theorem is known as the Law of Cosines and is usually used in trigonometry to solve problems related to triangles. It is a useful tool to calculate the length of a side or an angle in a triangle, given the lengths of the other sides and angles.

In the form of a formula:

If O is the midpoint of any side of a triangle, for a better understanding, consider O is the mid-point of side MN of a triangle LMN, according to Apollonius' Theorem.

Proof of Apollonius' Theorem

In general, we can prove this Theorem with the help of "Pythagoras' Theorem," but this Theorem can also be proved by two additional methods, which are "Using Coordinate Geometry and Vectors." So, in the following discussion, we learn about all these proofs one by one.

1. Proof by Pythagoras' Theorem

In a triangle ABC, let M be the midpoint of the BC side of the triangle ABC.

To Prove: AB2 + AC2 = 2 * [AM2 + CM2]

Proof: As we have,

Let us draw a perpendicular AH on the line BC, such that:

Now, according to the Pythagoras' Theorem, we have:

On adding equation (1) and equation (2), we get:

Add and Subtract 2*MH2 on the right side of the equation, we get:

From equation (3), we get:

As BM = CM = BC / 2, we get:

Which we have to prove, Hence Proved.

2. Proof by Considering Vectors

In a triangle, ABC, M is the midpoint of the side of triangle BC, such that Cartesian coordinates are defined as:

To Prove: AB2 + AC2 = 2 * [AM2 + CM2]

Proof: We have AB and AC equal to |b| and |c|, respectively. On putting these values on the left-hand side of the above equation, we get:

As BM = CM = BC / 2 (Because M is the midpoint of BC), we get:

Which we have to prove, Hence Proved.

3. Proof Using Coordinate Geometry

To prove Apollonius' Theorem, we have to consider O as an origin, which lies on the MN side of a triangle LMN, such that MN and OY are considered as the x-axis and y-axis, respectively, where MN = 2 * a and the coordinate points of M and N are (a, 0), and (-a, 0), respectively and coordinate of L is (b, c).

To Prove: (LM)² + (LN)² = 2 * [(LO)² + (MO)²]

Proof: As the coordinate point of O is (0, 0), LO is calculated as follows:

And squaring both sides, we get:

Similarly, LM is given by:

Similarly, MO is given by:

Similarly, LN is given by:

On adding equation (2) and equation (4), we get:

Which we have to prove, Hence Proved.

Applications of Apollonius' Theorem

While Apollonius' Theorem may appear abstract, its practical applications are found in various fields; some of them are discussed below:

  1. Geometric Analysis: Apollonius' Theorem is valuable for geometric analysis and problem-solving, especially problems that involve triangles; it allows mathematicians and engineers to better understand and manipulate the properties of triangles in various contexts.
  2. Engineering and Architecture: Engineers and architects use Apollonius' Theorem to design and analyze structures to ensure that load distribution and stability are properly distributed all over the structure.
  3. Computer Graphics: In computer graphics, understanding the relationships within triangles is essential for rendering 3-D models; Apollonius' Theorem helps in modeling and manipulating complex shapes.
  4. Surveying and Cartography: Surveyors and cartographers use Apollonius' Theorem when working with triangles on the surface of the Earth, helping to create accurate maps and land surveys.
  5. Statistics: In statistical analysis, Apollonius' Theorem can be adapted to explore relationships between data points in multi-dimensional space, contributing to data visualization and clustering algorithms.
  6. Triangle Analysis: Apollonius' Theorem provides a means to analyze triangles and calculate their circumradius, even when the angles and side lengths are unknown; this is valuable in various engineering and scientific fields, such as structural analysis and astronomy.
  7. Navigation: Apollonius' Theorem has applications in navigation, particularly in geodetic calculations, and it can be used to determine the radius of the circle that best fits a set of three non-collinear points on the Earth's surface, which is useful in surveying and mapmaking.
  8. Robotics: Robotics engineers use Apollonius' Theorem to analyze the geometry of robot manipulators and determine the optimal placement of sensors and actuators, and it plays a role in solving inverse kinematics problems.
  9. Molecular Chemistry: In the field of molecular chemistry, Apollonius' Theorem can be applied to analyze the arrangement of atoms in molecules, particularly in the context of molecular geometry.


Apollonius' Theorem, named after the ancient Greek mathematician Apollonius of Perga, is a powerful geometric result that relates the sides of a triangle to the median of that triangle; it is a live proof that shows its importance in mathematics. The proof of this Theorem, which can be solved with three different methods, shows the beauty of mathematical reasoning.

Beyond its theoretical importance, Apollonius' Theorem finds practical applications in diverse fields, from engineering and computer graphics to navigation and molecular chemistry. Its ability to bridge the gap between triangles and their median makes it a valuable tool for solving complex geometric problems and understanding the fundamental relationships in geometry. As we continue to explore the depths of mathematical knowledge, Apollonius' Theorem remains a valuable example of what mathematicians of history have given to us, reminding us that the beauty of mathematics increases with the increase in time and borders.

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