Primary 4 maths · Measurement & time · About 10 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.
Equal rows of squares explain area. Joining or removing surfaces changes how many squares are covered.
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 6 cm rectangle has a 1 cm square corner removed. Find the area of the remaining L-shape.
Identify the surface to cover and the lengths that determine its rows. Keep missing or overlapping pieces visible.
Area counts the surface covered by square units, with missing parts removed.
Dimensions describe the boundary and the covered surface. The full rectangle covers 5 × 6 = 30 square centimetres.
Dimensions describe the boundary and the covered surface. A 5 cm by 6 cm rectangle has a 1 cm square corner removed. Find the area of the remaining L-shape.
29 cm²
The full rectangle covers 5 × 6 = 30 square centimetres.
Dimensions describe the boundary and the covered surface. The missing square covers 1 × 1 = 1 cm².
Dimensions describe the boundary and the covered surface. 30 − 1 = 29 cm².
Dimensions describe the boundary and the covered surface. Check the result against the question we started with.
29 + 1 = 30.
We found 29 cm². Area counts the surface covered by square units, with missing parts removed.
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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