An illustration of a circle showing the circumference, centre, diameter and radius.

are completely round shapes.

Relationship with pi

Pi can be used to calculate the circumference of a circle, which is similar to the perimeter of other shapes. The circumference of a circle is always exactly pi (3.1415...) times the diameter of the circle, no matter how large or the small the circle is. For example, a circle with diameter 10 would have an area of 31.415...

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