1. Objectives
Write an equation for a circle.
Graph a circle, and identify its center and radius.
Vocabulary
circle tangent

2. Notes
1. Write an equation for the circle with center (1, –5) and a radius of
2. Write an equation for the circle with center (–4, 4) and containing the point (–1, 16).
3. Write an equation for the line tangent to the circle x2 + y2 = 17 at the point (4, 1).
4. Which points on the graph shown are within 2 units of the point (0, –2.5)? 2

3. A circle is the set of points in a plane that are a fixed distance, called the radius, from a fixed point, called the center. Because all of the points on a circle are the same distance from the center of the circle, you can use the Distance Formula to find the equation of a circle.

4. Example 1: Using the Distance Formula to Write the Equation of a Circle
Write the equation of a circle with center (–3, 4) and radius r = 6.
Use the Distance Formula with (x2, y2) = (x, y), (x1, y1) = (–3, 4), and distance equal to the radius, 6.
Use the Distance Formula.
Substitute.
Square both sides.

5. Notice that r2 and the center are visible in the equation of a circle
Notice that r2 and the center are visible in the equation of a circle. This leads to the formula for a circle with center (h, k) and radius r.
If the center of the circle is at the origin, the equation simplifies to x2 + y2 = r2.
Helpful Hint

6. Example 2A: Writing the Equation of a Circle
Write the equation of the circle.
the circle with center (0, 6) and radius r = 1
(x – h)2 + (y – k)2 = r2
Equation of a circle
(x – 0)2 + (y – 6)2 = 12
Substitute.
x2 + (y – 6)2 = 1

7. Example 2B: Writing the Equation of a Circle
Write the equation of the circle.
the circle with center (–4, 11) and containing the point (5, –1)
Use the Distance Formula to find the radius.
(x + 4)2 + (y – 11)2 = 152
Substitute the values into the equation of a circle.
(x + 4)2 + (y – 11)2 = 225

8. The location of points in relation to a circle can be described by inequalities. The points inside the circle satisfy the inequality (x – h)2 + (x – k)2 < r2. The points outside the circle satisfy the inequality (x – h)2 + (x – k)2 > r2.

9. Example 3: Consumer Application
Use the map and information given in Example 3 on page 730. Which homes are within 4 miles of a restaurant located at (–1, 1)?
The circle has a center (–1, 1) and radius 4. The points insides the circle will satisfy the inequality (x + 1)2 + (y – 1)2 < 42. Points B, C, D and E are within a 4-mile radius .
Check Point F(–2, –3) is near the boundary.
(–2 + 1)2 + (–3 – 1)2 < 42
(–1)2 + (–4)2 < 42 x
< 16
Point F (–2, –3) is not inside the circle.

10. A tangent is a line in the same plane as the circle that intersects the circle at exactly one point. Recall from geometry that a tangent to a circle is perpendicular to the radius at the point of tangency.

11. Example 4: Writing the Equation of a Tangent
Write the equation of the line tangent to the circle x2 + y2 = 29 at the point (2, 5).
Step 1 Identify the center and radius of the circle.
The circle has center of (0, 0)
and radius r = .
Step 2 Find the slope of the radius at the point of tangency and the slope of the tangent.
Use the slope formula.

12. Use the point-slope formula.
Example 4 Continued
Step 3 Find the slope-intercept equation of the tangent by using the point (2, 5) and the slope m = Perpendicular lines have slopes that are
opposite reciprocals 2 5 –
Use the point-slope formula.
Substitute (2, 5) (x1 , y1 ) and – for m. 2 5
Rewrite in slope-intercept form.

13. Example 4 Continued
The equation of the line that is tangent to x2 + y2 = 29 at (2, 5) is
Graph the circle and the line to check.

14. Notes
1. Write an equation for the circle with center (1, –5) and a radius of
(x – 1)2 + (y + 5)2 = 10
2. Write an equation for the circle with center (–4, 4) and containing the point (–1, 16).
(x + 4)2 + (y – 4)2 = 153
3. Write an equation for the line tangent to the circle x2 + y2 = 17 at the point (4, 1).
y – 1 = –4(x – 4)

15. Notes
4. Which points on the graph shown are within 2 units of the point (0, –2.5)?
C, F

16. Circles: Extra Info
The following power-point slides contain extra examples and information.
Reminder of Lesson Objectives
Write an equation for a circle.
Graph a circle, and identify its center and radius. 16

17. Write an equation for a circle.
Graph a circle, and identify its center and radius.
Vocabulary
circle tangent 17

18. Check It Out! Example 2
Find the equation of the circle with center (–3, 5) and containing the point (9, 10).
Use the Distance Formula to find the radius.
Substitute the values into the equation of a circle.
(x + 3)2 + (y – 5)2 = 132
(x + 3)2 + (y – 5)2 = 169

19. Check It Out! Example 4
Write the equation of the line that is tangent to the circle 25 = (x – 1)2 + (y + 2)2, at the point (1, –2).
Step 1 Identify the center and radius of the circle.
From the equation 25 = (x – 1)2 +(y + 2)2, the circle has center of (1, –2) and radius r = 5.

20. Check It Out! Example 4 Continued
Step 2 Find the slope of the radius at the point of tangency and the slope of the tangent.
Use the slope formula.
Substitute (5, –5) for (x2 , y2 ) and (1, –2) for (x1 , y1 ).
The slope of the radius is –3 4
Because the slopes of perpendicular lines are negative reciprocals, the slope of the tangent is .

21. Check It Out! Example 4 Continued
Step 3. Find the slope-intercept equation of the tangent by using the point (5, –5) and the slope
Use the point-slope formula.
Substitute (5, –5 ) for (x1 , y1 ) and for m. 4 3
Rewrite in slope-intercept form.

22. Check It Out! Example 4 Continued
The equation of the line that is tangent to 25 = (x – 1)2 + (y + 2)2 at (5, –5) is
Check Graph the circle and the line.