- Define and explain Perimeter and area in your own words
- Use key terms such as Perimeter accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
This lesson focuses on Perimeter and area: perimeter and area of rectangles, triangles, trapezia and compound shapes.
Perimeter and area of rectangles, triangles, trapezia and compound shapes.
Key ideas
Area formulae
Rectangle: base times height. Triangle: half times base times height, using the perpendicular height, not the slant side. Trapezium: half times the sum of the parallel sides times the distance between them.
Circles
Circumference equals pi times diameter (C = πd), and area equals pi times radius squared (A = πr²). Do not mix them up: circumference is a length, area is measured in square units.
Key term — Perimeter: The total distance around the edge of a 2D shape, found by adding all its side lengths.
Find the perimeter and area of a rectangle 7 cm by 4 cm.
Perimeter = 2(7 + 4) = 22 cm; area = 7 × 4 = 28 cm².
Answer: Perimeter = 2(7 + 4) = 22 cm; area = 7 × 4 = 28 cm².
- Forgetting units squared for area Area is measured in square units (cm²); perimeter is a length (cm). Always state the right unit.
- Confusing radius and diameter in circle formulae A = πr² uses the radius; C = πd uses the diameter. If given the diameter, halve it first for area.
Practice
Area = ½ × 10 × 6 = 30 cm².
Area = ½ × (8 + 5) × 4 = ½ × 13 × 4 = 26 cm².
A = 3.14 × 25 = 78.5 cm².
x = 180 − 112 = 68°.
Quick check
Which of these best defines "Perimeter"?
Find the circumference of a circle with diameter 14 cm (use π = 3.14).
- Perimeter and area: perimeter and area of rectangles, triangles, trapezia and compound shapes.
- Area formulae: Rectangle: base times height.
- Angle: The amount of turn between two lines that meet, measured in degrees.
- Watch out for: forgetting units squared for area