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Constructing Triangles


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Basics on the topic Constructing Triangles

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.

Learn more with this video on Constructing 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. 27212_ToV-03.svg

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) 27212_ToV-04.svg

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 cm, 2 cm, and 5 cm.


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


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?

If you are looking for more practice with the sum of angles in a triangle, check out this video: The Angle Sum Theorem

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?

Transcript Constructing Triangles

"Is it recording?" "Hey, it's Penny, from Penny's Perspective, coming at you with my latest DIY project building a tent for a sleepover!" Penny is sleeping at June's tonight, who is super knowledgeable with 'Constructing Triangles'. There are certain conditions that a triangle must meet. The angles inside the triangle must have a sum of exactly one hundred and eighty degrees. The sum of the two shortest sides must be greater than the length of the longest side. A plus B is greater than C. "First, get sticks for the two triangle legs, and the third side will be the floor." The blanket on the floor will be the third side and has a width of six feet. One stick is two feet long, and the other is three feet long. "Okay, June, will these two sticks work for our triangular tent?" Remember, the rule is the two shortest sides must add to more than the longest side. The two shortest sides are two feet and three feet, and when these sides are added together they have a sum of five feet. Five feet is less than the longest side, which is six feet, so these sticks will not work! "Bummer, looks like we will need longer sides!" New sticks are used, and they have lengths of two and four feet this time. Two plus four is equal to six and because six is not greater than six, this won't work! The sides will not reach to form a triangle unless they are greater than the longest side! Next, June will try two four-foot sticks, and the floor length is still six feet. Can a triangle be constructed with these side lengths? Pause the video to solve, and press play when you're ready for the solution. The sum is eight, which is greater than six. "Finally, we found sides that will work for our sleepover tent!" Remember though, all three interior angles of a triangle must add to one hundred and eighty degrees. Let's look at some options. Is it possible for a triangle to have angles of fifty, fifty, and sixty degrees? The sum is one hundred and sixty degrees, so it's not possible! What if the angles are forty, sixty, and eighty degrees? The sum is one hundred and eighty so, those could be the angle measures of a triangle. "Let's move this leg so the tent is even with two angles of fifty degrees." Pause the video here to determine the measurement of the last angle of the triangle, and press play when you are ready for the solution. The two angles we know are both fifty degrees, which has a sum of one hundred degrees All the angles must add up to one hundred and eighty degrees, so we need our last angle to be exactly eighty degrees! It looks like we have met all of the conditions that were needed to construct a triangular tent. In summary, there are two important conditions. The first is that the angles must have a sum of one hundred and eighty degrees. Also, the sum of the two shortest sides must be greater than the longest side. "Our sleepover tent is finally ready, thank you for watching Penny's Perspectives!"