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Fundamentals of Triangle Construction

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Learning text on the topic Fundamentals of Triangle Construction

Constructing Triangles

In geometry, constructing triangles isn't just about drawing shapes; it's a skill with real-world implications. Imagine an architect designing the roof of a house, where each section must form a perfect triangle for structural integrity. A small miscalculation in such a case can have costly consequences! This guide dives into the essential conditions for constructing triangles, focusing on practical examples to illustrate when a set of angles and sides can or cannot form a triangle.

Understanding Triangle Construction

To successfully construct a triangle, two key conditions must be met:

  1. The Sum of Two Sides Must be Greater Than the Third 27212_ToV-01.svg

  2. The Sum of Angles Equals 180 Degrees 27212_ToV-02.svg

Following these conditions ensures that the shape you create is a possible triangle.

Before diving further into this learning text, recap Different Types of Triangles.

Constructing Triangles – Step-by-Step Guide

To figure out if the triangle is possible, we use something called the Triangle Inequality Theorem. This rule says that if you add up the lengths of any two sides of a triangle, it should be greater than the length of the third side.

Let's look at an example, and make this easier with a step-by-step guide.

Imagine we have three sticks that are 4 cm, 7 cm and 10 cm long. To see if these can make a triangle, we add up the lengths of any two sticks and see if it's more than the length of the third stick.

Here's how we do it:

  • 4 cm + 7 cm = 11 cm (11 cm > 10 cm)
  • 4 cm + 10 cm = 14 cm (14 cm > 7 cm)
  • 7 cm + 10 cm = 17 cm (17 cm > 4 cm)

Since adding any two sticks together is always more than the length of the third stick, we can make a triangle with these sticks.


Now, let's try another example. This time we have sticks that are 3 cm, 5 cm and 9 cm long. Let's add them up:

  • 3 cm + 5 cm = 8 cm (8 cm < 9 cm)
  • 3 cm + 9 cm = 12 cm (12 cm > 5 cm)
  • 5 cm + 9 cm = 14 cm (14 cm > 3 cm)


In this case, adding the two shortest sticks (3 cm and 5 cm) is less than the length of the longest stick (9 cm). So, we can't make a triangle with these sticks.

Constructing Triangles – Examples

Let's try constructing a triangle with sides of 4 cm, 5 cm and 6 cm.

Triangle Measurements:

  • Side A: 4 cm
  • Side B: 5 cm
  • Side C: 6 cm
Do these side lengths meet the condition for constructing a triangle?
Can you suggest possible angle measurements for this triangle?
What tools would you need to accurately construct this triangle?

Example of an Impossible Triangle

Now, consider trying to make a triangle with sides of 2 ft, 2 ft and 5 ft.


  • Side A: 2 ft
  • Side B: 2 ft
  • Side C: 5 ft


Why can't a triangle be constructed with these side lengths?
If you tried to draw this triangle, what would go wrong?
What would happen if you tried to use these lengths to make a model of a triangle?

Triangle Construction – Summary

To be able to build a triangle, it is important to follow simple rules:

Possible Triangle Impossible Triangle
Side Lengths The sum of two sides is greater than the third side. The sum of two sides is less than or equal to the third side.
Angle Measurements The angles have a sum of 180°. The angles do NOT have a sum of 180°.
  • These rules are necessary to check if it's possible to create a triangle with the measurements provided.

Explore our platform for more resources on geometry, including interactive practice problems, instructional videos and printable worksheets to aid your learning journey in mathematics.

Constructing Triangles – Frequently Asked Questions

What is a triangle?
Why must the sum of two sides of a triangle be greater than the third side?
Can a triangle have more than 180 degrees?
What happens if the angles of a triangle don't add up to 180 degrees?
How do you measure the angles of a triangle?
What are the different types of triangles?
Can a triangle have all obtuse angles?
Is it possible to construct a triangle with any three line segments?
Why is understanding triangle construction important in geometry?
Can triangle construction principles be applied in real-life situations?

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