7-3 Circles p. 424

Formulas for circlesThe equation of a circle with center at (h, k) and radius r units is + = .

To graph or write the equation of a circle you need the following:

CenterRadius

Example 1A dog is tied in a yard using a 15-foot leash. The leash is anchored 6 feet south and 9 feet west of the dog’s food and water bowls. Assuming the bowls are at the origin, write an equation for the set of points at which the dog will be at the maximum length of the leash. Then draw the graph.

Example 3Write an equation of the circle that has its center at (2, -4) and is tangent to the x-axis. Tangent is defined as a line that intersects the circle in exactly one point.

Example 4Write an equation of a circle if the endpoints of a diameter are at (1, 8) and ( 1, -6).