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Measuring the outside boundary of shapes and comparing it to area.
If you wanted to build a fence for a new puppy, how would you know exactly how much wood to buy so the puppy doesn't escape?
Perimeter is the total distance around the outside edge of a flat shape. Imagine an ant crawling along the very edge of your notebook. If the ant starts at one corner and walks all the way around until it reaches the start again, the distance it traveled is the perimeter. To find this distance, we simply add the lengths of every single side together.
A square sticky note has four sides, and each side is inches long. 1. Identify the sides: There are 4 sides, each is inches. 2. Write the addition sentence: . 3. Solve: . The perimeter is inches.
Quick Check
If you are measuring the space 'inside' a shape, are you finding the perimeter?
Answer
No, perimeter is only the distance around the outside boundary.
Not all shapes have sides that are the same length. For a rectangle, the top and bottom are the same, and the left and right are the same. For a triangle, you might have three different numbers. The rule never changes: Add every side! A good trick is to mark each side with a small 'x' after you add it so you don't count the same side twice.
A garden is feet long and feet wide. 1. List all four sides: . 2. Set up the sum: . 3. Group the numbers: . 4. Calculate: . The perimeter is feet.
Quick Check
A triangle has sides of cm, cm, and cm. What is its perimeter?
Answer
cm
Sometimes, we know the total perimeter but one side is a mystery! We can use our detective skills to find it. First, add up all the sides you do know. Then, subtract that total from the full perimeter. The number left over must be the length of the missing side.
A triangle has a total perimeter of inches. You know two sides are inches and inches. How long is the third side? 1. Add the known sides: inches. 2. Look at the total: The whole distance must be inches. 3. Subtract to find the difference: . The missing side is inches.
What is the perimeter of a square where each side is cm?
A rectangle has sides of inches and inches. What is the perimeter?
To find the perimeter of any shape, you should multiply the side lengths together.
Review Tomorrow
Tomorrow morning, look at your TV or a window and try to name the four sides you would add to find its perimeter.
Practice Activity
Find a book and use a ruler to measure all four sides. Add them up to find the book's total perimeter!