If you know the perimeter and some of the sides, you can find a missing side. For a rectangle, half the perimeter is one length plus one width.
A rectangle has a perimeter of 30 cm and a length of 9 cm. Half of 30 is 15, so length + width = 15. The width is 15 − 9 = 6 cm. A square has four equal sides, so each side is the perimeter divided by 4.
Worked example
A square has a perimeter of 36 cm. How long is each side?
- A square has 4 equal sides.
- 36 ÷ 4 = 9.
Answer:9 cm
Key words
- missing side
- a side length we must work out
- half
- divide by 2
- square
- a rectangle with 4 equal sides
Practise this
New questions every time.
Common Core-referenced: 3.MD.D.8