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Perimeter and area
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Perimeter is the distance around a shape. Area is the space inside it. Each common shape has its own area formula.
Perimeter
Add up all the sides. A rectangle 8 cm by 5 cm has perimeter 8 + 5 + 8 + 5 = 26 cm.
Perimeter is a length, so it is in cm, m and so on.
Perimeter is a length, so it is in cm, m and so on.
Area formulae
Area is measured in square units such as cm2.
Compound shapes
Split the shape into rectangles and triangles, or find a big rectangle and subtract the missing piece.
An L-shape inside a 10 × 8 rectangle with a 4 × 3 corner cut out has area 80 − 12 = 68.
An L-shape inside a 10 × 8 rectangle with a 4 × 3 corner cut out has area 80 − 12 = 68.
A trapezium has parallel sides 5 cm and 9 cm and height 4 cm. Find its area.
- 12 × (5 + 9) × 4
- = 12 × 14 × 4
Answer: 28 cm2
Perimeter: add the sides. Area: rectangle l × w, triangle ½bh, parallelogram bh, trapezium ½(a + b)h. Always use the perpendicular height.
The interactive lesson includes the diagrams for this topic.
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