Circles
The radius goes from the centre to the edge; the diameter goes all the way across, so it is twice the radius. The distance round the edge is the circumference.
Key facts
- Diameter = 2 × radius.
- Circumference C = πd (or 2πr).
- Area A = πr².
- π ≈ 3.14. Use the π button for exact answers.
Worked examples
Example 1
A circle has radius 7 cm. Find its area to 1 decimal place.
- Square the radius first. 7² = 49
- Multiply by π. π × 49 = 153.938…
- Round. 153.9 cm²
Answer: 153.9 cm²
Example 2
A wheel has a diameter of 10 cm. How far does it roll in one turn?
- One turn rolls the circumference. C = π × 10
- Work it out. C ≈ 31.4 cm
Answer: 31.4 cm