Objective: Students will find the perimeter of triangles. They will use their understanding of the 4 operations to answer questions about perimeter and missing side lengths.
Vocabulary: triangle, side, length, inch, centimeter, measure, ruler, perimeter
What is perimeter?
Perimeter is a linear measurement. Perimeter measures the distance around the sides of a shape. Think of the perimeter as the boundary or line around the shape.
To find the perimeter of a triangle,
add each side length together.
Think of perimeter as measuring a shape’s outside line if it were straightened out.
To measure the perimeter of triangles you need to know its 3 side lengths. The side lengths are added together to find the perimeter.
What is the perimeter of the triangle?
Always check your unit of measure!
This triangle is measured in inches.
The perimeter of the triangle is 36 inches.
9 inches + 12 inches + 15 inches = 36 inches
Finding Perimeter of Equilateral Triangles
Remember this triangle from earlier? The fancy name for this kind of triangle is “equilateral” because its sides are all equal.
How can you find the perimeter of this triangle with only one side length?
If I know the side lengths are equal, I can multiply 12 cm times 3.
12 cm + 12 cm + 12 cm = 36 cm and
12 cm x 3 sides = 36 cm
Finding the perimeter with a missing side
You may also be given a problem with a missing side length. If you are given the perimeter and 2 sides, you can find the missing side by subtracting.
What operation can you use to find the missing side length?
I can add and subtract to find the missing side length.
Another way to look at this problem is as addition with a missing addend.
24 meters + 24 meters + ? = 82 meters
48 meters + ? = 82 meters
82 – 48 = 34, so the missing side length is 34 meters!
What is the missing side length if the perimeter of the triangle is 24 feet?
Always check your unit of measure! This triangle is measured in feet.
10 feet
6 ft + 8 ft + ? = 24 feet
14 ft + ? = 24 feet
24 is 10 more than 14, so the missing side length is 10 feet long.
Finding the perimeter with 3 missing sides
You may also be given a problem with 3 missing side lengths. If you know the side lengths are equal, you can still solve the problem!
What operation can you use to find the missing side lengths?
Division
If you know the 3 triangle sides are equal, you can find the side lengths by dividing the perimeter by 3.
45 cm ÷ 3 = 15 cm
What are the side lengths of the triangle if they are all equal?
21 centimeters
If you know the 3 triangle sides are equal, you can find the side lengths by dividing the perimeter by 3.
63 cm ÷ 3 = 21 cm
Measuring Perimeter Error Analysis
What’s the mistake?
There are two mistakes in this solution!
First of all, all 3 sides weren’t added.
50 + 50 + 56 = 156, not 106.
Second of all, the triangle measurements are given in yards, not meters.
The perimeter of the triangle is 156 yards.
Closing Questions
- What does each triangle have in common?
- Which unit of measures have perimeters that are much larger than the units you worked with on the math mats today?
- Describe the process for finding a missing side length when you know the perimeter.
Resources to Teach Perimeter
The lesson from this blog post is part of our 3rd Grade Math Curriculum. Want more lessons like this one? Click here to view the editable mini-lessons.
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