GoalUse the words square, rectangle, side and perimeter correctly and find the perimeter by adding the sides.
New words
perimeter · the distance all the way around a shapeside · one straight edge of a shapesquare · a shape with 4 equal sides and 4 square cornersrectangle · a shape with 4 square corners and opposite sides equal
Explanation
A square has 4 equal sides and 4 square corners. A rectangle has 4 square corners and its opposite sides are equal, so its length is equal on the top and the bottom and its width is equal on the two ends. Every square is also a rectangle, but a rectangle is a square only when all its sides are equal. The perimeter is the distance all the way around a shape. To find it, add the lengths of all the sides. For a square with sides of 6 cm, the perimeter is 6 + 6 + 6 + 6 = 24 cm, which is 4 × 6. Write the perimeter in units of length, such as cm or m, not square units.
Worked examples
A square has sides of 6 cm. Add the four sides: 6 + 6 + 6 + 6 = 24. The perimeter is 24 cm, which is 4 × 6.
A rectangle is 9 cm long and 4 cm wide. Its sides are 9, 4, 9 and 4, so the perimeter is 9 + 4 + 9 + 4 = 26 cm.
Side
Length
top
12 m
right
5 m
bottom
12 m
left
5 m
A garden is a rectangle 12 m long and 5 m wide. A fence all the way round needs 12 + 5 + 12 + 5 = 34 m. A shortcut is to double the length and the width and add: 24 + 10 = 34 m.
Class activity
Perimeter walk. Pick a rectangular thing at home, such as a table, a book or a door. Measure its length and width with a ruler or a tape. Add all four sides to find its perimeter. Is it longer or shorter than 1 metre?
Practice
1
A square has sides of 7 cm. What is its perimeter?
4 × 7 = 28 cm.
2
A rectangle is 10 cm long and 3 cm wide. Find its perimeter.
10 + 3 + 10 + 3 = 26 cm.
3
This school yard is a rectangle. Find the perimeter.
8 + 5 + 8 + 5 = 26 m.
4
A square has a perimeter of 36 cm. How long is one side?
The 4 sides are equal, so one side is 36 ÷ 4 = 9 cm.
5
Marwan says a rectangle 6 cm long and 2 cm wide has a perimeter of 12 cm, because 6 × 2 = 12. What is wrong?
He multiplied, which does not give the perimeter. The perimeter is 6 + 2 + 6 + 2 = 16 cm.
