lesson
Trace every outside edge
1 · Learn
A rectilinear shape has straight sides meeting at right angles. To find its perimeter, trace the outside and add every boundary length once.
For an L-shape, imagine a 6 cm by 4 cm rectangle with a 2 cm wide, 1 cm deep corner removed from the top right. Its boundary lengths are 4, 1, 2, 3, 6 and 4 cm. Their total is 20 cm.
2 · Worked example
The two short edges around the missing corner replace the removed top and right pieces. I include them because they are part of the new outside boundary.
3 · Your turn
Draw a 7 cm by 5 cm rectangle. Find its perimeter. If a 2 cm by 2 cm top-right corner is removed, trace the new boundary and find its perimeter.
Check your answer
Both perimeters are 24 cm. The removed 2 cm top and right lengths are replaced by two 2 cm indentation lengths. This is specific to this corner removal, not every cut-out.
4 · Apply your learning
Draw the L-shape on squared paper and label the boundary before adding. Do not count an interior construction line.
Adult guidance and safety
Use practical or drawn representations and explain the method. Check units and labels. A correct answer to one example is not evidence of secure fluency; revisit with varied numbers and feedback.
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