Perimeter of Obtuse Triangle
2026-02-21 21:13 Diff
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Last updated on December 28, 2025

The perimeter of a shape is the total length of its boundary. Specifically, the perimeter of an obtuse triangle is the sum of all three sides. Perimeter is also used for fencing a property, sewing, and more. In this topic, we will learn about the perimeter of an obtuse triangle.

What is the Perimeter of an Obtuse Triangle?

The perimeter of an obtuse triangle is the total length of its three sides. By adding the length of all three sides, we get the perimeter of the shape.

The formula for the perimeter of an obtuse triangle is 𝑷 = 𝑎 + 𝑏 + 𝑐, where a, b, and c are the sides of the triangle.

For instance, if an obtuse triangle has sides, a = 8, b = 15, and c = 20, then its perimeter is P = 8 + 15 + 20 = 43.

Formula for Perimeter of Obtuse Triangle - 𝑷 = 𝑎 + 𝑏 + 𝑐.

Let’s consider another example of an obtuse triangle with side lengths, 𝑎 = 7, 𝑏 = 24, and 𝑐 = 25.

So the perimeter of the triangle will be: 𝑷 = 𝑎 + 𝑏 + 𝑐 = 7 + 24 + 25 = 56.

How to Calculate the Perimeter of an Obtuse Triangle

To find the perimeter of an obtuse triangle, we just need to apply the given formula and sum all the sides of the triangle.

For instance, a given obtuse triangle has the sides of a = 9, b = 12, c = 15. Perimeter = sum of all sides = 9 + 12 + 15 = 36 cm.

Example

Problem on Perimeter of Obtuse Triangle - For finding the perimeter of an obtuse triangle, we use the formula, 𝑷 = 𝑎 + 𝑏 + 𝑐.

For example, let’s say, a = 10 cm, b = 17 cm, and c = 21 cm. Now, the perimeter = sum of all sides = 10 + 17 + 21 = 48 cm.

Therefore, the perimeter of the obtuse triangle is 48 cm.

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Tips and Tricks for Perimeter of Obtuse Triangle

Learning some tips and tricks makes it easier for children to calculate the perimeter of obtuse triangles. Here are some tips and tricks given below:

  • Always remember that an obtuse triangle's perimeter is simply the sum of the three sides of the shape. For that, use the formula, 𝑷 = 𝑎 + 𝑏 + 𝑐.
  • Calculating the perimeter of an obtuse triangle starts by determining the length of each side using the distance formula. The distance formula is: Distance = √((x2-x1)²+(y2-y1)²). Here, (x1, y1) and (x2, y2) indicate the positions of two points that make out a triangle’s side. They can be found by adding the lengths of three sides after they are calculated.
  • To reduce confusion, specifically arrange the indicated side lengths if you need the perimeter of a group of triangles. After that, apply the formula to each triangle.
  • To avoid mistakes when adding the perimeter, make sure the side lengths are precise and constant for common uses like gardening and architecture.
  • If you are given the semi-perimeter, which is half the perimeter, you can multiply it by 2 to determine the full perimeter. Area-related calculations, like Heron’s formula, often use the semi-perimeter.

Common Mistakes and How to Avoid Them in Perimeter of Obtuse Triangle

Did you know that while working with the perimeter of an obtuse triangle, children might encounter some errors or difficulties? We have many solutions to resolve these problems. Here are some given below:

Problem 1

A triangular garden has a perimeter of 50 meters and two sides measuring 18 meters and 15 meters. To find out the missing side, subtract the sum of the known sides from the total perimeter.

Okay, lets begin

Length of the missing side = 17 meters.

Explanation

Let ‘c’ be the missing side. And the given perimeter = 50 meters.

Lengths of the two sides = 18 meters and 15 meters.

Perimeter of triangle = sum of lengths of three sides. 50 = 18 + 15 + c

50 = 33 + c

c = 50 – 33 = 17

c = 17

Therefore, the missing side is 17 meters.

Well explained 👍

Problem 2

A wire with a perimeter of 150 inches is reshaped into an obtuse triangle. Find the length of each side of the triangle if the sides are in the ratio of 3:4:5.

Okay, lets begin

Lengths are 45 inches, 60 inches, and 75 inches.

Explanation

Given that the perimeter of the triangle is equal to the total length of the wire and the sides are in the ratio of 3:4:5, here is the solution:

Let the sides be 3x, 4x, and 5x.

Perimeter of the triangle = 3x + 4x + 5x = 150

12x = 150

x = 12.5

Therefore, the lengths are 3x = 37.5, 4x = 50, and 5x = 62.5. However, to ensure integer values, let's use 9x, 12x, 15x for a similar ratio: 3x = 45, 4x = 60, 5x = 75.

Hence, the lengths are 45 inches, 60 inches, and 75 inches.

Well explained 👍

Problem 3

Find the perimeter of an obtuse triangle whose sides are 11 cm, 13 cm, and 17 cm.

Okay, lets begin

41 cm

Explanation

Perimeter of triangle = a + b + c P = 11 + 13 + 17 = 41

Therefore, the perimeter of the triangle is 41 cm.

Well explained 👍

Problem 4

Mark is designing a triangular banner in the shape of an obtuse triangle. He measures the three sides of the banner: Side A = 14 meters Side B = 18 meters Side C = 22 meters How much ribbon should Mark use to outline the banner?

Okay, lets begin

Mark will need 54 meters of ribbon to outline the banner.

Explanation

The perimeter of a triangle is the sum of all the three sides.

Using the formula: P = a + b + c

P = 14 + 18 + 22 = 54 meters.

Well explained 👍

Problem 5

Find the perimeter of an obtuse triangular-shaped plot of land.

Okay, lets begin

Sides are a = 16, b = 30, c = 34

Perimeter = a + b + c = 16 + 30 + 34 = 80 meters.

Explanation

Each side of the obtuse triangle has a different length. The entire distance calculated around the plot of land is 80 meters by summing the lengths of the three sides.

Well explained 👍

FAQs on Perimeter of Obtuse Triangle

1.Evaluate the perimeter of an obtuse triangle if its sides are 7 cm, 24 cm, and 25 cm.

Perimeter of triangle = a + b + c, Hence P = 7 + 24 + 25 = 56 cm.

2.What is meant by an obtuse triangle’s perimeter?

The total length around an obtuse triangle’s sides is its perimeter. In other words, the perimeter of an obtuse triangle is the total length of its sides.

3.What are the types of obtuse triangles?

Obtuse triangles can be scalene, isosceles, or equilateral depending on their side lengths, but they always have one angle greater than 90°.

4.Which triangle has an obtuse angle?

An obtuse triangle has one angle that is greater than 90°, making it an obtuse angle.

5.Can an obtuse triangle be equilateral?

No, an equilateral triangle cannot be obtuse because all its angles are equal to 60°, which are acute angles.

Important Glossaries for Perimeter of Obtuse Triangle

  • Perimeter: The total length of the sides of a shape.
  • Obtuse Triangle: A triangle in which one of the angles is greater than 90°.
  • Formula of Perimeter: The mathematical expression used to calculate the perimeter of an obtuse triangle is 𝑷 = 𝑎 + 𝑏 + 𝑐.
  • Side Lengths: The measurements of each side of a triangle.
  • Distance Formula: A mathematical formula used to compute the length between two points in a plane: Distance = √((x2-x1)²+(y2-y1)²).

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Seyed Ali Fathima S

About the Author

Seyed Ali Fathima S a math expert with nearly 5 years of experience as a math teacher. From an engineer to a math teacher, shows her passion for math and teaching. She is a calculator queen, who loves tables and she turns tables to puzzles and songs.

Fun Fact

: She has songs for each table which helps her to remember the tables