## Circle

A circle is the set of points in a plane that are equidistant from one point. That one point is called the center of the circle, and the distance from it to any point on the circle is called the radius of the circle. The standard form for the equation of a circle with its center at (0, 0) and with a radius of length r is represented by the equation

x 2 + y 2 = r 2

##### Example 1

Graph x 2 + y 2 = 16.

Recognize that x 2 + y 2 = 16 is the equation of a circle centered at (0, 0) with r 2 = 16. So r = 4, as shown in Figure 1.

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