Mathematics · Grades 7–10
Circumference of a circle
The diameter is twice the radius.
The key idea
The diameter is twice the radius.
C = 2πr = πd
Worked examples
A circle has radius 25 cm. Its circumference is kπ cm. Find k.
C = 2 × π × 25 = 50π
50
A circle has radius 35 cm. Its circumference is kπ cm. Find k.
C = 2 × π × 35 = 70π
70
Try it yourself
A circle has radius 60 cm. Its circumference is kπ cm. Find k.
A circle has radius 36 cm. Its circumference is kπ cm. Find k.
Check your understanding
Answers and marking notes
- 120
C = 2 × π × 60 = 120π
- 72
C = 2 × π × 36 = 72π
Further reading and review references
These are reference links, not copied exercises, institutional endorsement or proof of independent review.
Learning preview · teacher review required. Grade bands are approximate, not official curriculum alignment.