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Dan Harmon says
The area of a circle is pi*r*r, where pi is approximately 3.1415 and r is the radius of the circle, or 1/2 of the diameter.

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ankigarg87 says
Yhe area of a disk (the region inside a circle) is ?r2 when the circle has radius r. Here the symbol ? (Greek letter pi) denotes, as usual, the constant ratio of the circumference of a circle to its diameter.

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