Circumference and Area of a Circle

To calculate the circumference (which is like perimeter) and the area of a circle, you need a special irrational number: pi. Pi is the ratio of the circumference of a circle to its diameter. Watch this lecture series to explore the circumference and area of circles, and complete the interactive exercises.

Radius, diameter, circumference & π


Source: Khan Academy, https://www.khanacademy.org/math/cc-seventh-grade-math/cc-7th-geometry
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