3.2.2 PERIMETER OF PLANE FIGURES
Perimeter is the distance around a figure. The table below shows the formulae used to find the perimeter of different plane figures.
Example 3.5
A rectangle has a length of 8 cm and a width of 4 cm. Calculate its perimeter.
Solution
P = 2l + 2w
P = 2 × 8 + 2 × 4
P = 16 + 8
P = 24 cm
Example 3.6
The figure below has a perimeter of 74 cm. Calculate the length of the side marked x.
Solution
12 cm + 12 cm + 12 cm + 12 cm + 12 cm + x = 74 cm
60 cm + x = 74 cm
x = 74 cm − 60 cm
x = 12 cm
Compare Perimeters Activity (Grade 7)
Enter measurements so the triangle, rectangle, and square all have the same perimeter. Then click Compare Perimeters.
Triangle
Rectangle
Square
Enter values, then click the button.
Exercise
Use the Check Answers tab to enter answers. Use the Show Working tab to view the steps for each question.
P = 2(l + w).- Perimeter of a rectangle:
P = 2(l + w) - Substitute:
100 = 2(35 + w) - Divide by 2:
50 = 35 + w w = 50 − 35 = 15- Width = 15 cm
- Perimeter of a square:
P = 4s - Substitute:
3496 = 4s - Divide by 4:
s = 3496 ÷ 4 = 874 - Side = 874 m
- Use Pythagoras:
l² + w² = d² - Substitute:
l² + 6² = 10² l² + 36 = 100sol² = 64l = √64 = 8- Perimeter:
P = 2(l + w) = 2(8 + 6) = 28 - Perimeter = 28 cm
- Perimeter of a rectangle:
P = 2(l + w) - Substitute:
84 = 2(25 + w) - Divide by 2:
42 = 25 + w w = 42 − 25 = 17- Width = 17 cm
- Circumference:
C = 2πr C = 2 × (22/7) × 35 = 220cm- Distance in 20 revolutions:
20 × 220 = 4400cm - Convert to metres:
4400 ÷ 100 = 44m - Distance = 44 m
- Perimeter:
P = 2(l + w) - Substitute:
P = 2(40 + 18) P = 2 × 58 = 116- Perimeter = 116 m