Equations of Circles

You can write an equation of a circle in a coordinate

plane, if you know:

• Its radius• The coordinates of its center

Standard Equation of a Circle:

222 rkyhx

kh, is the center of the circle

r is the radius of the circle.

If the center is the origin, then the standard

equation is

222 ryx

To graph a circle on a coordinate plane:

1. Identify the center2. Graph the center3. Identify the radius

4. Start at the center and count the number of units that the radius is above the center, below the center, to the left of the center and to the right of the center.

5. Draw the circle to connect those points.