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Pi is represented by the symbolπ(the sixteenth letter of the Greek alphabet) and appears in a number of measurement formulas. These formulas include *c* = 2*πr* and *A=πr*^{2}, where *c* is a circle's circumference or the distance around its outer edge, *r* is its radius or the distance from the center of the circle to its outer edge, and *A* is the circle's area. What is pi and why doesπappear in these formulas?

## What Is Pi?

What do the following items have in common with each other and all other circular figures: a quarter, approximately 1 inch across; a compact disc, 12 centimeters wide; a circular patio pool, spanning about 23 feet; and Earth's equator, roughly 8,000 miles in **diameter**. These estimated diameters are summarized in the table below, along with the estimated circumferences of the circular items.

In each case in...

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