25 Circles in theCoordinate Plane
0: Mathematics Florida StandardsMAFS.912.G-GPE.1.1 Derive the equation of a circlegiven center and radius using the PythagoreanTheorem...
MP1.MP3,MP4, MP7
Objectives To write the equation of a circleTo find the center and radius of a circle
MATHEMATICAL
PRACTICES
Getting Ready!
Do you need tocheck the distance
to every part ofthe course?
The owners of an outdoor odventure
course wont a way to communicate
to all points on the course. Theyore considering purchosing wolkie-totkies with o range of | mi. Amodel of the course is at the right.Each grid unit represents mi. Thebose stotion is ot (2, 4). Do you thinkthe owners should buy the wolkie-tolkies? Why?
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Lesson ^Vocabularystandard form ofan equation of acircle
In the Solve It, all of the obstacles lie witliin or on a circle with the base station as the
center. The information from the diagram is enough to write an equation for the circle.
Essential Understanding The information in the equation of a circle allows youto graph the circle. Also, you can write the equation of a circle if you know its centerand radius.
Theorem 12-16 Equation of a Circle
An equation of a circle with center {h, k) and radius r is(a: — h)^ + {y — k)^ — r^.
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{h,k) j' d X
798 Chapter 12 Circles
How is this problemdifferent from
Problem 1?
In this problem, youdon't know r. So the first
step is to find r.
Here's Why It Works You can use the Distance Formula to findan equation of a circle with center [h, k) and radius r, which provesTheorem 12-16. Let {x, y) be any point on the circle. Then the radius ris the distance from [h, k) to {x, y).
d = V(x2 — x{)^ + (y2 ~ Distance Formula
r= \^{x — + {y — k)^ Substitute (x, y) for (X2,y2) and {h, k) for(xi,yi).
r^ = {x- hf + {y- k)^ Square both sides.
The equation (x - + (y - k)^ = is the standard form of an equation of acircle. You may also call it the standard equation of a circle.
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{h. k) jo' X
What do you needto know to write the
equation of a circle?You need to know the
values of h, k, and r.h is the x-coordinate
of the center, k is they-coordinate of thecenter, and r is the radius.
Writing the Equation of a Circle
What is the standard equation of the circle with center (5, — 2) and radius 7?
{x- hf + (y- k)^ - Use the standard form of an equation of a circle,
(x: — 5)^ + [y — (—2)]^ = 7^ Substitute (5, -2) for {h, k) and 7 for r.
[x — 5)^ + (y + 2)^ = 49 Simplify.
Gofit? 1 . What is the standard equation of each circle?a. center (3,5); radius 6 b. center (—2, -1); radius
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Problem 2 Using the Center and a Point on a Circle
What is the standard equation of the circle with center
(1, — 3) that passes through the point (2,2)?
Step 1 Use the Distance Formula to find the radius.
r = \/(x2 — -Vi)^ + (y2 ~ Distance Formula.
= V(1 -2)2-i-{-3-2)2 Substitute (1, -3) for (X2,y2)and (2,2)for(xi,yi).
Simplify.= V'(-1)2 + (-5)2
=
step 2 Use the radius and the center to write an equation.
(x — -F (y - kf- = Use the standard form of an equation of a circle,
(x — 1)^ + [y - (-3)]^ = (V56)^ Substituted, -3) for (h, k) and V56 forr.
(x—1)^ + (y + 3)^ = 26 Simplify.
Got It? 2. What is the standard equation of the circle with center (4, 3) that passesthrough the point (-1,1)?
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C PowerGeometry.com Lesson 12-5 Circles in the Coordinate Plane 799
If you know the standard equation of a circle, you can describe the circle by naming its
center and radius. Then you can use this information to graph the circle.
Problem 3 Graphing a Circle Given Its Equation 52D
Communications When you make a call on a cell phone, a tower receives and
transmits the call. A way to monitor the range of a cell tower system is to use
equations of circles. Suppose the equation {x — 7)^ + (y + 2)^ = 64 represents theposition and the transmission range of a cell tower. What is the graph that shows the
position and range of the towerV
The equation representingthe cell tower's positionand range
To draw a graph Determine the values of (h, k) and r inthe equation. Then draw a graph.
(x - if + (y + 2)2 = 64(x - 7)2 + [y - (-2)] = 82
t V th k r
Use the standard equation of a circle.
Rewrite to find h, k, and r.
The center is (7, —2) and the radius is 8.
To graph the circle, place the compass point at the center(7, —2) and draw a circle with radius 8.
Got It? 3. a. In Problem 3, what does the center of the
circle represent? What does the radius
represent?
b. What is the center and radius of the circle
with equation (x — 2)2 + (y — 3)2 = 100?Graph the circle.
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Lesson Check
Do you know HOW?What is the standard equation of each circle?
1. center (0, 0); r = 4
2. center (1, -1); r= Vs
What is the center and radius of each circle?
3. (x-8)2+y2 = 9
4. (x + 2)2 + (y - 4)2 = 7
__ MATHEMATICAL
Do you UNDERSTAND? IWf PRACTICES5. What is the least amount of information that you
need to graph a circle? To write the equation of acircle?
6. Suppose you know the center of a circle and a pointon the circle. How do you determine the equation of
the circle?
7. Error Analysis A student says that the center of acircle with equation (x — 2)2 + (y + 3)2 = 16 is(-2,3). What is the student's error?
800 Chapter 12 Circles
Practice and Problem-Solving Exercises
Practice Write the standard equation of each circle.
MATHEMATICAL
PRACTICES
^ Apply
8. center (2, —8); r = 9 9. center (0,3); r = 7
11. center (5, — 1); r = 12 12. center (—6,3); r = 8
14. center (0, 0); r = 4 15. center (-4,0); r = 3
Write a standard equation for each circle in the diagram at the right.
17. QP 18. 0Q
Write the standard equation of the circle with the given center that
passes through the given point.
19. center (—2, 6); point (—2,10) 20. center (1,2); point (0, 6)
21. center (7, -2); point (1, -6) 22. center (-10, -5); point (-5,5}
23. center (6, 5); point (0, 0) 24. center (—1, -4); point (-4,0)
Find the center and radius of each circle. Then graph the circle.
^ See Problem 1.
10. center (0.2,1.1); r = 0.4
13. center (—9, —4); r = Vs
16. center (—1, —1); r = 1
^ See Problem 2.
^ See Problem 3.
25. (x+7)2+(y-5)2 _ 16
27. {x + 4)'^ + (y - 1)2 = 25
26. (x-3)2+(y+8)2-
28. x2+j/2 = 36
100
Public Safety Each equation models the position and range of a tornado alertsiren. Describe the position and range of each.
29. (x - 5)2 + (y - 7)2 81 30. (x + 4)2 + (y - 9)2 = 144
Write the standard equation of each circle.
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Lesson 12-5 Circles In the Coordinate Plane 801
Write an equation of a circle with diameter AB.
37. >1(0, 0), B(8, 6) 38. >1(3. 0), B(7, 6) 39. >1(1,1), B(5, 5)
40. Reasoning Describe the graph of + y^ = p- when r = 0.
Determine whether each equation is the equation of a circle. Justify
your answer.
41. (x- 1)2 + (y+ 2)2 = 9 42. x + y=9 43. x + (y - 3)^ = 9
44. Think About a Plan Find the circumference and area of the circle whose equation
is (x — 9)2 + (y — 3)2 = 64. Leave your answers in terms of tt.• What essential information do you need?
• What formulas will you use?
45. Write an equation of a circle with area 3077 and center (4,7).
46. What are the x- and y-intercepts of the line tangent to the circle
(x — 2)2 + (y — 2)2 = 52 at the point (5,6)?
47. For (x - Ji)2 + (y - k)^ = r^, show that y = Vr^-[x-h)^ + fc or
y= - Vr2-(x-h)2 + A:.
Sketch the graphs of each equation. Find all points of intersection of each pair
of graphs.
48.x2+y2=l3 49.x2 + y2=l7 50.x2 +y2 = 8
y = -X + 5 y=-\x y = 2
51. x2+y2 = 20 52. (x+ 1)2 + (y - 1)2 := I8 53. (x - 2)2 + (y - 2)2 = 10
y = —|x + 5 y = x + 8 y = —^x + 654. You can use completing the square and factoring to find the center and
radius of a circle.
a. What number c do you need to add to each side of the equation
x2 + 6x + y2 — 4y = —4 so that x2 + 6x + c can he factored intoa perfect square binomial?
b. What number d do you need to add to each side of the equation
x2 + 6x + y2 — 4y = —4 so that y2 — 4y + rf can he factored intoa perfect square binomial?
c. Rewrite x2 + 6x + y2 — 4y = —4 using your results from parts (a) and (h).d. What are the center and radius of x2 + 6x + y2 — 4y = —4?e. What are the center and radius of x2 + 4x + y2 — 20y 4-100 = 0?
Challenge 55. The concentric circles (x - 3)2 + (y — 5)2 = 64 and (x — 3)2 + (y — 5)2 = 25form a ring. The lines y = |x + 3 and y = 5 intersect the ring, making foursections. Find the area of each section. Round your answers to the nearest tenth of
a square unit.
802 Chapter 12 Circles
SAT/Aa
Short
.ResponseNmIT
56. Geometry in 3 Dimensions The equation of a sphere is similar to theequation of a circle. The equation of a sphere with center [h,j, k] and radius
ris (x — h)^ + (y + (z — k)^ = r^. M(—1, 3, 2) is the center of asphere passing through T(0,5,1). What is the radius of the sphere? What isthe equation of the sphere?
57. Nautical Distance A close estimate of the radius of Earth's equator is 3960 mi.
a. Write the equation of the equator with the center of Earth as the origin.
b. Find the length of a 1° arc on the equator to the nearest tenth of a mile.c. History Columbus planned his trip to the East by going west. He thought each
1° arc was 45 mi long. He estimated that the trip would take 21 days. Use your
answer to part (b) to find a better estimate.
Standardized Test Prep
58. What is an equation of a circle with radius 16 and center (2, -5)?
CS> {_x-2f + {y+ 5)2 = 16 CO (x + 2)2 + (y - 5)2 = 256
CO (x + 2)2 + (y - 5)2 = 4 <0> (x - 2)2 + (y + 5)2 = 256
59. What can you NOT conclude from the diagram at the right?
CO c = d CO a = b
CO c2 + e2 = 2 e = d
60. Are the following statements equivalent?
• In a circle, if two central angles are congruent, then they have congruent arcs.
• In a circle, if two arcs are congruent, then they have congruent central angles.
Mixed Review
Find the value of each variable. ^See Lesson 12-4.
218°
Get Ready! To prepare for Lesson 12-6/ do Exercises 63-65.
Sketch each of the following.
63. the perpendicular bisector of BC
64. line k parallel to line m and perpendicular to line w, all in plane N
65. Z-£FG bisected by FH
4^ See Lessons 1-2 and 1-5.
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Lesson 12-5 Circles in the Coordinate Plane 803