Primary 5 Foundation maths · Measurement & time · About 12 minutes
Find areas and perimeters of composite rectilinear figures
Identify the surface to cover and the lengths that determine its rows. Keep missing or overlapping pieces visible.
Start the interactive lesson →Open practice activity
What you will learn
- Find areas and perimeters of composite rectilinear figures
Models used: number sentences, stories, diagrams.
Before you start
Split the shape into familiar pieces
Imagine the full 6 by 4 rectangle. It would cover 24 square units.
Imagine the full 6 by 4 rectangle. It would cover 24 square units.
The complete rectangle. A 2 by 2 corner is missing. That missing square covers 4 square units.
A square corner has been removed. Only count the area that is still there: 24 minus 4 is 20.
6 × 4 − 2 × 2 = 20
The remaining L-shape covers twenty square units.
What did the picture help us see?
Only count the area that is still there: 24 minus 4 is 20.
Worked example: A new example, together
A 5 cm by 7 cm rectangle has a 1 cm square corner removed. Find the perimeter of the remaining L-shape.
Identify the surface to cover and the lengths that determine its rows. Keep missing or overlapping pieces visible.
Perimeter counts the boundary once, including inward edges but excluding internal joins.
Dimensions describe the boundary and the covered surface. Trace only the outer boundary.
Dimensions describe the boundary and the covered surface. A 5 cm by 7 cm rectangle has a 1 cm square corner removed. Find the perimeter of the remaining L-shape.
24 cm
The corner removes two short edges and adds two equal-length inner edges, leaving the total boundary length unchanged.
Dimensions describe the boundary and the covered surface. Perimeter = 2 × (5 + 7) = 24 cm.
Dimensions describe the boundary and the covered surface. Check the result against the question we started with.
Trace the complete boundary once and add every outer segment.
We found 24 cm. Perimeter counts the boundary once, including inward edges but excluding internal joins.
Dimensions describe the boundary and the covered surface.
Common mistakes to discuss
- Using the outside measurements only. Split the figure into parts, find each part, then combine. A missing side is found from the sides facing it.
Check your understanding
- Find areas and perimeters of composite rectilinear figures
- explain area calculations using covered square units
- check the answer against the original quantities and units
Explain your reasoning to an adult, then try a fresh question in the interactive lesson. A result from one attempt is not proof of lasting mastery.
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