Find a circle’s exact circumference
The idea
Identify the radius, which runs from the center to the circle.
Step by step
- Identify the radius, which runs from the center to the circle.
- Double the radius to find the diameter, then multiply that length by π.
- Keep π in the answer for an exact value, and use length units rather than square units.
Example
A circle has radius 2 cm. What is its exact circumference?
Answer
4π cm
The diameter is 2 × 2 = 4 cm. Circumference is π times diameter, so C = 4π cm.
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Wrap string around a circular object, compare the circumference with its diameter and estimate their ratio.
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