Theorem 7.8 Class 9 – Sum of two sides of a triangle is greater than

Triangles are fundamental shapes in geometry, and understanding their properties is crucial in various mathematical applications. In Class 9, students come across Theorem 7.8, which affirms that the total of any two sides of a triangle will invariably exceed the length of the third side. This theorem provides a fundamental principle for analyzing and solving problems related to triangles. In this blog post, we will explore Theorem 7.8 in detail, discussing its proof, implications, and practical applications.

The Sum of Any Two Sides of a Triangle is Greater than the Third Side

The Importance of Theorem 7.8 Class 9

Theorem 7.8 Class 9 is a fundamental concept in triangle geometry, and it forms the basis for many other theorems and properties related to triangles. Understanding this theorem allows students to confidently analyze and solve problems involving triangle inequalities. It is an essential tool in geometry, providing a framework for proving and understanding various geometric relationships.

Understanding Triangle Inequalities

Before we delve into the proof of Theorem 7.8, let’s first understand the concept of triangle inequalities. In any triangle, the sum of the lengths of any two sides must always be greater than the length of the third side. This principle holds true for all types of triangles, including scalene, isosceles, and equilateral triangles.

The Proof of Theorem 7.8

To prove Theorem 7.8, we start with a triangle ABC and extend one of its sides, say AB, to a point D such that AD is equal to AC. Now, we have triangle ACD. By the triangle inequality theorem, the sum of the lengths of any two sides of a triangle must be greater than the length of the third side. Applying this theorem to a triangle ACD, we have AD + AC > CD.

Implications of Theorem 7.8

The implications of Theorem 7.8 are far-reaching. It allows us to analyze and solve various problems related to triangles. For example, it helps us determine whether a given set of side lengths can form a valid triangle. If the sum of the lengths of any two sides is less than or equal to the length of the third side, then such a triangle cannot exist. This insight is particularly useful in real-world applications, such as engineering and architecture, where the validity of geometric structures must be assessed.

Practical Applications of Theorem 7.8

Theorem 7.8 finds applications in various fields, including engineering, architecture, and physics. Engineers rely on the theorem to ensure the stability and structural integrity of bridges and buildings. Architects use it to determine the feasibility of constructing certain geometric shapes. Physicists apply Theorem 7.8 in analyzing forces and vectors in a triangular system, helping them understand the equilibrium and stability of physical objects.

Example Problems

To further illustrate the applications of Theorem 7.8, let’s consider a few examples of problems.

Example 1: Determining Triangle Validity

Given three side lengths, a, b, and c, we can determine whether a triangle with those side lengths is valid by applying Theorem 7.8. If a + b > c, a + c > b, and b + c > a, then the triangle is valid. Otherwise, it is not possible to form a triangle with those side lengths.

Example 2: Finding Unknown Side Lengths

Suppose we are given a triangle with known side lengths a, b, and c, and we are asked to find the length of an unknown side. By rearranging Theorem 7.8, we can solve the unknown side length. For example, if we know that a + b > c, we can solve for c by subtracting side length a from both sides of the inequality: c > a + b.


Theorem 7.8 Class 9 holds significant importance in the realm of triangle geometry as it establishes that the total of any two sides of a triangle will consistently surpass the length of the third side. Understanding this theorem allows students to analyze and solve problems related to triangles, and it has practical applications in various fields. By applying Theorem 7.8, we can determine triangle validity, find unknown side lengths, and analyze the stability of geometric structures. This theorem forms the foundation for further exploration of triangle geometry and its applications in real-world scenarios.

Additional Information:

  • The proof of Theorem 7.8 involves extending a side of the triangle to create a new triangle and applying the triangle inequality theorem.
  • The theorem is also known as the Triangle Inequality Theorem.
  • The concept of triangle inequalities can be extended to higher dimensions, such as quadrilaterals and polygons.