1. 10.6 Equations of Circles
Geometry

2. What do you need to find the equation of a circle?
Coordinates of the Center of the circle.
Radius – Distance from center to a point on the circle

3. Let’s Look at a Circle

4. Break it Down
Given: Center is (h,k), the radius is r, and (x,y) is a point on the circle. The distance between (x,y) and (h,k) is r… so let’s use the distance formula.

5. Break it Down

6. Standard Equation of a Circle
(x – h)² + (y – k)² = r² Where the center is at (h,k) & The radius is r

7. Example
Write the standard equation of a circle that has a center at the origin and a radius of 5.

8. Example
Write the standard equation of a circle with center (-5, 0) and radius 4.8.

9. Example
The point (2,1) is on a circle whose center is (4,-3). Write the standard equation of the circle.

10. Example
The point (0, -1) is on a circle whose center is (-4, 1). Write the standard equation of the circle.

11. Graphing
The equation of a circle is (x + 2)² + (y – 3)² = 9. First find the center and the radius of the circle, and then graph the circle.

12. Graphing
Graph (x – 3)² + (y + 1)² = 4