1. GeometryGeometry Equations of Circles

2. GeometryGeometry Finding Equations of Circles You can write an equation of a circle in a coordinate plane if you know its radius and the coordinates of its center.

3. GeometryGeometry Equation for a Circle An equation of a circle with center (h, k) & radius (r): (x – h) 2 + (y – k) 2 = r 2 r (h, k) (x, y)

4. GeometryGeometry Ex.1: Write the Equation of a circle. Write the standard equation of a circle with center (-8, 1) and radius of √ 5. Graph it. (x – h) 2 + (y – k) 2 = r 2 (x – (-8)) 2 + (y – 1) 2 = ( √ 5) 2 (x + 8) 2 + (y – 1) 2 = 5 -8 1 √5√5

5. GeometryGeometry Ex.2: Find the center and radius of a circle with the following equation. Equation: (x – 4) 2 + (y + 2) 2 = 25 –Center (4, -2) –Radius = 5

6. GeometryGeometry Ex.3: Graph the circle with the following Equation (x + 4) 2 + (y – 1) 2 = 36 –Center: (-4, 1) –Radius: 6 (x – (-4)) 2 + (y – 1) 2 = ( √ 36) 2 -4 1 2

7. GeometryGeometry Ex.4: Write the equation of a circle from its graph. 4 (x – h) 2 + (y – k) 2 = r 2 (x – 0) 2 + (y – 0) 2 = 4 2 x 2 + y 2 = 16

8. GeometryGeometry Ex.4: More Circles Write the standard equation of a circle with center (5, 8) & passes through point (-15, -13). –Step 1: Solve for r –Step 2: Put into standard equation (x – h) 2 + (y – k) 2 = r 2 (-15 – 5) 2 + (-13 - 8) 2 = r 2 -20 2 + -21 2 = r 2 400 + 441 = r 2 841 = r 2 29 = r hk xy (x – h) 2 + (y – k) 2 = r 2 (x – 5) 2 + (y – 8) 2 = 29 2 (x – 5) 2 + (y – 8) 2 = 841

9. GeometryGeometry Ex. 1: Writing a Standard Equation of a Circle Write the standard equation of the circle with a center at (-4, 0) and radius 7.1 (x – h) 2 + (y – k) 2 = r 2 Standard equation of a circle. [(x – (-4)] 2 + (y – 0) 2 = 7.1 2 Substitute values. (x + 4) 2 + (y – 0) 2 = 50.41 Simplify.

10. GeometryGeometry Ex. 2: Writing a Standard Equation of a Circle The point (1, 2) is on a circle whose center is (5, -1). Write a standard equation of the circle. r = r = 5 Use the Distance Formula Substitute values. Simplify. Addition Property Square root the result.

11. GeometryGeometry Ex. 2: Writing a Standard Equation of a Circle The point (1, 2) is on a circle whose center is (5, -1). Write a standard equation of the circle. (x – h) 2 + (y – k) 2 = r 2 Standard equation of a circle. [(x – 5)] 2 + [y –(-1)] 2 = 5 2 Substitute values. (x - 5) 2 + (y + 1) 2 = 25 Simplify.