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12.04 Equations of circles

Adaptive
Worksheet
What do you remember?
1

State the equation for a circle with center \left(h,k\right) and radius r.

2

Consider the graph of the circle shown.

Copy and complete the following statement:

Every point on the circle is exactly ⬚ units away from the point (⬚ ,⬚).

-2
-1
1
2
3
4
5
6
x
-5
-4
-3
-2
-1
1
2
3
y
3

Consider circle with center C and \triangle ABC with given side lengths. Find the radius of the circle using the Pythagorean Theorem.

x
y
4

Find the distance between Point A and Point B rounded to two decimal places.

a

A \left(1, 4\right) and B \left(7, 12\right)

b

A \left(4, 2\right) and B \left( - 8 , - 7 \right)

c

A \left( - 1 , 9\right) and B \left( - 4 , 1\right)

d

A \left(-1, - \dfrac{3}{5} \right) and B \left(4, \dfrac{12}{5}\right)

Let's practice
5

For each of the following circle graphs:

i
State the coordinates of the center.
ii
State the radius of the circle.
iii
State the diameter of the circle.
iv
State the equation of the circle.
a
-5
-4
-3
-2
-1
1
2
3
4
5
x
-5
-4
-3
-2
-1
1
2
3
4
5
y
b
-3
-2
-1
1
2
3
4
5
x
-6
-5
-4
-3
-2
-1
1
2
y
c
-1
1
2
3
4
5
6
7
x
-4
-3
-2
-1
1
2
3
4
y
d
-8
-6
-4
-2
2
4
6
x
-6
-4
-2
2
4
6
8
10
y
6

For each of these equations:

i

Find the center of the circle.

ii

Find the radius of the circle.

iii

Graph the circle.

a
x^{2} + y^{2} = 16
b
\left(x + 4\right)^{2} + \left(y - 2\right)^{2} = 25
7

For each of the these equations for circles:

i

State the center of the circle.

ii

Calculate the diameter of the circle.

a
(x-2)^2+(y+3)^2=75
b
(x-2.5)^2+y^2=24.01
c
x^2+(y-4)^2=13
d
\left(x+\dfrac{1}{2}\right)^2+\left(y-\dfrac{5}{4}\right)^2=\dfrac{9}{4}
8

For each of the these descriptions, write the equation of the circle.

a

A circle with center at (3,\,3) and a radius of 6 units.

b

A circle with center at \left(0,\,-6\right) and a diameter of 6 units.

c

A circle with center at \left(\dfrac{17}{2},\,0\right) and a diameter of 2\sqrt{15} units.

d

A circle with center at \left(\dfrac{1}{2},\,-\dfrac{2}{3}\right) and a radius of \dfrac{5}{4} units.

9

Using the given points, find the equations for the circles:

a
Center at \left(-2,5\right), passing through the point \left(3,-7\right)
b
Center at \left(0,0\right), passing through the point \left(5,-10\right)
c
Center at \left(0.5,-2\right), passing through the point \left(5,\,6.4\right)
d
Diameter with endpoints at \left(4,4\right),and \left(-2,6\right)
e
Diameter with endpoints at \left(0,-2\right), and \left(13.6,\,0.4\right)
f

Diameter with endpoints at \left(10, 6\right) and \left( - 12 , - 14 \right)

10

Given: Circle P with center at \left(-3,5\right).

Which equation could represent circle P?

A

\left(x - 3\right)^2 + \left(y - 5\right)^2 = 41

B

\left(x - 3\right)^2 + \left(y + 5\right)^2 = 41

C

\left(x + 3\right)^2 + \left(y - 5\right)^2 = 41

D

\left(x + 3\right)^2 + \left(y + 5\right)^2 = 41

11

A circle has a center \left(4,\,0 \right) and goes through the point \left(3,\,-2 \right). Which of these could be the equation of the circle?

A
\left(x - 4\right)^2 + y^2 = 5
B
\left(x + 4\right)^2 + y^2 = 5
C
\left(x - 4\right)^2 - y^2 = 5
D
x^2 + \left(y-4 \right)^2 = 5
12

Fill in the blanks to complete the equation of the circle.

Circle Q with diameter \overline{GH}: G(-5,\,3) and H(3,\,-1).

Create the equation of this circle.

The Equation of the Circle⬚+⬚=⬚
(x+1)^2(x-1)^2(y-1)^2(y+1)^22050
13

A circle has a center at ( 3,\, -5) and a radius of 6 units. Create the equation of this circle.

The Equation of the Circle⬚⬚⬚=⬚
(x-3)(x+3)
(x-3)^2(x+3)^2
(x-5)(x+5)
(x-5)^2(x+5)^2
+-
3^26^2
14

Find the equation of each circle:

a
-3
-2
-1
1
2
3
4
5
6
7
x
-5
-4
-3
-2
-1
1
2
3
4
5
y
b
-5
-4
-3
-2
-1
1
2
3
4
5
x
-1
1
2
3
4
5
6
7
8
9
y
c
-3
-2
-1
1
2
3
4
5
6
7
x
-7
-6
-5
-4
-3
-2
-1
1
2
3
y
d
-1
1
2
3
4
5
6
7
8
9
10
11
x
-1
1
2
3
4
5
6
7
8
9
10
11
y
15

Determine whether the given points are inside, outside, or on the circle with equation x^{2} + y^{2} = 25.

a
(- 3,\,2)
b
(4,\,3)
c
(1,\,6)
d
(-5,\,0)
16

Circle O has a center at (-1,\,3) and a diameter of 10 units.

Which point lies on circle O?

A

(-6,\,-5)

B

(2,\,7)

C

(16,\,4)

D

(8,\,8)

17

Which point lies on the circle represented by the equation (x-3)^2 + (y-5)^2 = 5^2?

A

(0,\,1)

B

(0,\,4)

C

(-1,\,5)

D

(8,\,3)

18

A wind turbine has 20\text{ m} long blades that are attached to a tower 30\text{ m} high. The distance from the origin to the base of the wind turbine is 25\text{ m}.

Find the standard form of the equation of the circle represented by the path of the blades of the wind turbine.

A figure of a wind turbine imposed on a first quadrant coordinate plane without numbers. The wind turbine has 3 identical blades with a length of 20 meters. The horizontal distance between the origin and the center of the turbine's base is 25 meters. The vertical distance between the rotor and the x axis is 30 meters.
19

The circle shown has a radius of 5. Explain how the Pythagorean theorem can be used to derive the equation for this circle.

x
y
Let's extend our thinking
20

A soccer match is being televised.

One of the cameras is mounted on a drone which is programmed to zoom in on the ball only when it is inside the center circle.

The drone uses a coordinate system to track the position of the ball, where the origin is at the bottom left corner of the field and each unit corresponds to 1\text{ m}.

A diagram of a soccer field that has a length of 99 meters, and width of 75 meters. The center circle of the soccer field has a radius of 9.15 meters.
a

The center of the center circle is in the exact center of the field. Find the coordinates of the circle's center.

b

State the equation for the circle.

c

State the domain and range of the circle in interval notation.

21

A circle has a diameter with endpoints of A\left(-1,\,1\right) and B\left(9,\,-7\right).

a

Determine the equation of the circle.

b

Prove that the point P\left(8,\,3\right) is outside of the circle.

22

Coralee needs to use her school’s laser cutter to make a cork component for her design project.

Using technology to sketch the component, she knows that the intersection of two different sized circles inscribed on a piece of cork gives the shape that she wants.

Two overlapping circles in a rectangle. A dashed line connects the centers of the circles. The region where the two circles overlapped is shaded.

Two such circles have the equations \left(x - 12\right)^{2} + \left(y - 8\right)^{2} = 49 and

\left(x - 20\right)^{2} + \left(y - 8\right)^{2}= 36, where each unit is 1\text{ cm} and \left(0,\,0\right) is the bottom left corner of the given cork board.

a

State the domain and range of the leftmost circle in interval notation.

b

State the domain and range of the rightmost circle in interval notation.

c

Find the width of the component that Coralee wants to make.

d

Find the least area of material that could be used to make Coralee's shape, considering the following constraints:

  • The laser cutter will only accept rectangular pieces of material.

  • The height of the intersection is 10.17\text{ cm}.

  • There also needs to be a gap of at least 1\text{ cm} between any cut and the edge of the piece.

23

Three receiving stations are located on a coordinate plane, where each unit is one mile, at the points \left( - 3,\,- 2 \right),\,\left( - 1,\,2\right) and \left(3, 1\right). The epicenter of an earthquake is determined to be 2 miles, 4 miles and 5 miles from these three points, respectively.

a

Determine the coordinates of the epicenter of the earthquake.

b

The earthquake can be felt 10 miles away. Determine if the town located at \left(8,\,-7\right) would feel the earthquake.

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Outcomes

G.PC.4

The student will solve problems in the coordinate plane involving equations of circles.

G.PC.4bi

Solve problems in the coordinate plane involving equations of circles: i) given a graph or the equation of a circle in standard form, identify the coordinates of the center of the circle;

G.PC.4bii

Solve problems in the coordinate plane involving equations of circles: ii) given the coordinates of the endpoints of a diameter of a circle, determine the coordinates of the center of the circle.

G.PC.4biii

Solve problems in the coordinate plane involving equations of circles: iii) given a graph or the equation of a circle in standard form, identify the length of the radius or diameter of the circle.

G.PC.4bv

Solve problems in the coordinate plane involving equations of circles: v) given the coordinates of the center and the coordinates of a point on the circle, determine the length of the radius or diameter of the circle; and

G.PC.4bvi

Solve problems in the coordinate plane involving equations of circles: vi) given the coordinates of the center and length of the radius of a circle, identify the coordinates of a point(s) on the circle.

G.PC.4ci

Determine the equation of a circle given: i) a graph of a circle with a center with coordinates that are integers;

G.PC.4cii

Determine the equation of a circle given: ii) coordinates of the center and a point on the circle;

G.PC.4ciii

Determine the equation of a circle given: iii) coordinates of the center and the length of the radius or diameter;

G.PC.4civ

Determine the equation of a circle given: iv) coordinates of the endpoints of a diameter.

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