Perimeter and area · Usually met around P4
Perimeter of composite shapes
A side not marked: use the side opposite.
How to do it
Method
A side not marked: use the side opposite
An L shape's perimeter = 2 × (length + width)
Another way
Add every side
10 + 4 + 3 + 3 + 7 + 7 = 34
In full
The perimeter of a composite shape is every outside edge added up. For an edge with no length marked, use the sides opposite: in an L shape the two top pieces together equal the long bottom, and the two right pieces equal the left side. So the L shape's perimeter equals the big rectangle's: 2 × (length + width). To find a side from the perimeter, work backwards: a square's side is the perimeter ÷ 4, and a rectangle's width is the perimeter ÷ 2 − the length. When two rectangles are put together, the edge they share is inside and doesn't count. Or check by writing every side out and adding.
Worked examples
Have a go first, then open each one for the answer and the working.
Example 1
A square's perimeter is 12 cm. How long is each side, in cm?
Show the answer and working
Answer: 3 cm
A square's four sides are equal: 12 ÷ 4 = 3.
Example 2
Lengths are in cm, and two sides have no length marked. What is the perimeter of the shape in cm?
Show the answer and working
Answer: 46 cm
The two top pieces equal the bottom, 13, and the two right pieces equal the left, 10: 2 × (13 + 10) = 46.
Example 3
Two rectangles 15 cm by 9 cm are put together along their long sides to make a bigger rectangle. What is its perimeter in cm?
- A. 78
- B. 96
- C. 66
- D. 48
Choose the right answer
Show the answer and working
Answer: C. 66
The big rectangle is 15 cm by 2 × 9 cm: 2 × (15 + 2 × 9) = 66.
Practise it in the game →Perimeter and area · Free to start, unlimited questions, no download