☰ Year plan · Mathematics

Breaking arrays and perimeter

Week 10 · Breaking arrays and perimeter · 3 lessons
2

Measuring sides and perimeter

Discover Mathematics P3: Term 1, Lessons 31–40
GoalMeasure the sides of a shape with a ruler and add them to find the perimeter.
New words
perimeter · the distance all the way around a shapeside · one straight edge of a shapeunit · what we measure with, such as cmstring · a thin cord we can lay around a shape to measure its edge
Explanation

The perimeter of a shape is the distance all the way around it. To find it, measure every side with a ruler, write each length with its unit, and add all the sides. You can check with a string: lay the string around the edge, then straighten it and measure it with the ruler. A rectangle with sides of 5 cm and 3 cm has four sides 5, 3, 5 and 3, so its perimeter is 5 + 3 + 5 + 3 = 16 cm. Perimeter is a length, so we write cm.

5 cm3 cm
Worked examples
7 cm4 cm3 cm4 cm
This trapezium has sides of 7 cm, 4 cm, 3 cm and 4 cm. The perimeter is 7 + 4 + 3 + 4 = 18 cm.
6 cm4 cm
This parallelogram has two sides of 6 cm and two sides of 4 cm. The perimeter is 6 + 4 + 6 + 4 = 20 cm.
5 cm
Marwan has a string 20 cm long. It goes exactly around a square with sides of 5 cm. The four equal sides give 5 + 5 + 5 + 5 = 20 cm, or 4 × 5 = 20 cm.
Class activity

Perimeter with string. Choose three flat things at home: a book, a plate lid, a photo frame. Lay a string around the edge of each, mark the place where it meets, straighten it and measure it with a ruler. Write the perimeter with its unit. For the book, also measure the four sides and add them. Do you get the same answer?

Practice
1
Find the perimeter of this rectangle.
9 cm4 cm
2
A square tile has a side of 7 cm. What is its perimeter?
3
A four-sided shape has sides of 5 cm, 8 cm, 6 cm and 9 cm. What is its perimeter?
4
A rectangle has a perimeter of 24 cm and a length of 8 cm. What is its width?
5
Hana says the perimeter of a rectangle with sides of 6 cm and 2 cm is 12 cm, because 6 × 2 = 12. What is her mistake?