63
Perimeter of a rectangle
Textbook: Part 2, lesson 63, pp. 75–77
GoalFind the perimeter of a rectangle with the formula P = 2 × (a + b).
New words
perimeter · perimetrlength and width · bo‘y va enformula · formulaside · tomon
Explanation
The perimeter is the sum of the lengths of all sides of a shape. Opposite sides of a rectangle are equal, so P = a + b + a + b = 2 × (a + b), where a is the length and b the width. For example, with sides 4 cm and 8 cm, P = 2 × (4 + 8) = 24 cm = 2 dm 4 cm. To find an unknown side, subtract the known side from the half-perimeter (P : 2).
Worked examples
A 15 m × 9 m garden has perimeter P = 2 × (15 + 9) = 48 m.
If P = 50 cm and a = 18 cm, then b = 50 : 2 − 18 = 25 − 18 = 7 cm.
Class activity
“Building a fence”: children measure the classroom floor or a desk and find its perimeter in two ways (adding four sides and with the formula).
Practice
1
A rectangle is 12 cm long and 7 cm wide. Find its perimeter (cm).
38
2
A farmer’s field is 300 m long and 150 m wide. How many metres of wall are needed to surround it?
900
3
A rectangle has perimeter 64 dm and length 20 dm. What is its width in dm?
12
4
A room is 4 m 50 cm long and 3 m wide. What is its perimeter in metres?
15
5
A rectangle has perimeter 50 cm and one side 18 cm. Find the other side (cm).
7