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Last updated on September 26, 2025

Perimeter of Isosceles Triangle

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

Perimeter of Isosceles Triangle for US Students
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What is the Perimeter of an Isosceles Triangle?

The perimeter of an isosceles triangle is the total length of its three sides. In an isosceles triangle, two sides are of equal length, and the third side is different.

 

By adding the length of all three sides, we get the perimeter of the shape. The formula for the perimeter of an isosceles triangle is ( P = 2a + b ), where ( a ) is the length of the equal sides, and ( b ) is the base of the triangle.

 

For instance, if an isosceles triangle has equal sides ( a = 6 ) and base ( b = 8 ), then its perimeter is ( P = 2 times 6 + 8 = 20 ).

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Formula for Perimeter of Isosceles Triangle - \( P = 2a + b \).

Let’s consider another example of an isosceles triangle with equal side lengths, ( a = 10 ), and base ( b = 12 ).

 

So the perimeter of the isosceles triangle will be: ( P = 2a + b = 2 times 10 + 12 = 32 ).

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How to Calculate the Perimeter of an Isosceles Triangle

To find the perimeter of an isosceles triangle, we just need to apply the given formula and sum the two equal sides and the base of the triangle. For instance, a given isosceles triangle has the sides of ( a = 7 ), ( b = 5 ). Perimeter = sum of all sides = ( 2 times 7 + 5 = 19 ) cm.

 

Example Problem on Perimeter of Isosceles Triangle 

 

For finding the perimeter of an isosceles triangle, we use the formula, ( P = 2a + b ).

 

For example, let’s say, ( a = 9 ) cm, ( b = 4 ) cm.

 

Now, the perimeter = sum of all sides = ( 2 times 9 + 4 = 22 ) cm.

 

Therefore, the perimeter of the isosceles triangle is 22 cm.

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Tips and Tricks for Perimeter of Isosceles Triangle

Learning some tips and tricks makes it easier for children to calculate the perimeter of isosceles triangles. Here are some tips and tricks given below: Always remember that an isosceles triangle's perimeter is simply the sum of the two equal sides and the base. For that, use the formula, ( P = 2a + b ).

 

Calculating the perimeter of an isosceles triangle starts by determining the length of each side using the distance formula. The distance formula is: Distance = ( sqrt{(x_2-x_1)2+(y_2-y_1)2} ). Here, ( (x_1, y_1) ) and ( (x_2, y_2) \) indicate the positions of two points that make up a side of the triangle.

 

They can be found by adding the lengths of the three sides after they are calculated. To reduce confusion, specifically, arrange the indicated side lengths if you need the perimeter of a group of isosceles 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.

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

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

Mistake 1

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Incorrectly using the Pythagorean theorem (( a2 + b2 = c2 )) for triangles that are not right-angled.

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Remember that only right-angled triangles can use the Pythagorean theorem.

 

The lengths of all the sides of other triangle types should be summed up to find the perimeter.

Mistake 2

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Students may confuse the concepts of area and perimeter, leading to incorrect calculations.

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It is important to note that the perimeter, calculated in linear units, is the total length of the triangle’s sides, while the area, in square units, represents the space inside the triangle.

Mistake 3

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Assuming that every triangle is isosceles or equilateral, which results in inaccurate calculations.

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A scalene triangle has all the sides of different lengths.

 

Always be careful about the type of the triangle to ensure accurate calculations.

Mistake 4

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Using the wrong values for the length of the sides, misreading a problem, or mislabeling the sides.

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Double-check the problem and ensure that you're using the correct lengths for your triangle.

Mistake 5

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Misunderstanding or misidentifying the side lengths that are given in a problem outline.

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Before starting the calculations, make sure that you understand the problem and, if it's possible, try to draw a triangle to accurately identify the side lengths.

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Perimeter of Isosceles Triangle Examples

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Problem 1

A triangular-shaped garden has a perimeter of 40 meters and two sides measuring 15 meters each. To find out the missing side of the garden, subtract the sum of the known sides from the total perimeter.

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Length of the missing side = 10 meters.

Explanation

Let ‘b’ be the length of the missing side.

And the given perimeter = 40 meters.

Length of the two equal sides = 15 meters.

Perimeter of isosceles triangle = sum of lengths of three sides.

40 = 15 + 15 + b 40 = 30 + b b = 40 – 30 = 10

Therefore, the missing side is 10 meters.

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Problem 2

A piece of string with a perimeter of 90 inches is reshaped into an isosceles triangle with equal sides measuring 35 inches each. Find the length of the base of the triangle.

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20 inches

Explanation

Given that the perimeter of the string is equal to the total length of the triangle, here is the solution:

Perimeter of the isosceles triangle = 2a + b 90 = 2 times 35 + b 90 = 70 + b b = 90 - 70 = 20

Therefore, the length of the base of the triangle is 20 inches.

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Problem 3

Find the perimeter of an isosceles triangle whose equal sides are 9 cm and base is 12 cm.

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30 cm

Explanation

Perimeter of isosceles triangle = 2a + b ( P = 2 \times 9 + 12 = 30 )

Therefore, the perimeter of the triangle is 30 cm.

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Problem 4

Mark is designing a triangular flower bed in his backyard. He measures the three sides of the garden: Equal Side A = 8 meters Equal Side B = 8 meters Base C = 6 meters How much material should Mark buy to go around the edge of the flower bed?

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Mark will need 22 meters of material to go around the garden.

Explanation

The perimeter of an isosceles triangle is the sum of the two equal sides and the base.

Using the formula: ( P = 2a + b ) ( P = 2 times 8 + 6 = 22 ) meters.

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Problem 5

Find the perimeter of an isosceles triangular field where the equal sides are 13 meters each and the base is 10 meters.

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Perimeter = 2a + b = 2 times 13 + 10 = 36 meters.

Explanation

Each equal side of the isosceles triangle is 13 meters, and the base is 10 meters.

The entire distance around the field is calculated to be 36 meters by summing the lengths of the three sides.

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FAQs on Perimeter of Isosceles Triangle

1.Evaluate the isosceles triangle’s perimeter if its equal sides are 7 cm and base is 5 cm.

Perimeter of isosceles triangle = 2a + b, Hence \( P = 2 \times 7 + 5 = 19 \) cm.

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

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

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3.What are the types of triangles?

There are six types of triangles: Equilateral triangle, Isosceles triangle, Scalene triangle, Acute triangle, Right triangle, and Obtuse triangle.

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4.Which triangle has two equal sides?

An isosceles triangle is a triangle with two equal sides.

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5.Which triangle has the smallest perimeter given equal side lengths?

For given side lengths, an equilateral triangle will have the smallest perimeter compared to other types of triangles with different arrangements.

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Important Glossaries for Perimeter of Isosceles Triangle

  • Perimeter: The total length of the sides of a shape.

 

  • Isosceles Triangle: A triangle with two sides of equal length and one side of different length.

 

  • Base: The unequal side of an isosceles triangle.

 

  • Equal Sides: The two sides of an isosceles triangle that have equal lengths.

 

  • Formula of Perimeter for Isosceles Triangle: The mathematical expression used to calculate the perimeter of an isosceles triangle is \( P = 2a + b \).
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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.

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

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