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Circles

Circumference, area and the meaning of π.

π is the ratio of any circle's circumference to its diameter — the same number for a coin and for a planet. From the radius alone you get the diameter (2r), the circumference (2πr) and the area (πr²).

Worked example: circle radius 2

Geometry · Circles

Circle radius 2

2

Step by step

  1. r = 2

    Everything about a circle follows from its radius.

  2. d = 2r = 4

    Diameter.

  3. C = 2\pi r = 4 \pi \approx 12.566

    Circumference.

  4. A = \pi r^2 = 4 \pi \approx 12.566

    Area.

Reveal the answer
C = 4 \pi,\quad A = 4 \pi

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