The rectangle shortcut
A postcard: 7 cm across, 4 cm tall. Must all four sides be added?
Step 1 of 5
- A postcard, 7 cm by 4 cm.
- The two long sides match: 7 cm each.
- The two short sides match: 4 cm each.
- Add all four: 7 + 4 + 7 + 4 = 22 cm.
- Faster: 2 × (7 + 4) = 22 cm.
The rule
No. Add the two different sides, then double.
A rectangle has four sides, but only two lengths. The longer side is the length. The shorter side is the breadth. Opposite sides, the ones facing each other, always match.
Walking the edge still works: length + breadth + length + breadth. Length and breadth each appear twice. So add them once, then double.
Perimeter = 2 × (length + breadth)
For the postcard: 2 × (7 cm + 4 cm) = 2 × 11 cm. That's 22 cm. Same answer, just faster.
Watch out!
7 × 4 = 28 is not the perimeter. Add the two sides first. Then double.
Slide the sides. Adding all four sides and the shortcut give one answer.
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