# EQUATIONS OF CIRCLES WORKSHEET

## About "Equations of Circles Worksheet"

Equations of Circles Worksheet :

Worksheet given in this section is much useful to the students who would like to practice problems on equations of circles.

Before look at the worksheet, if you would like to know the stuff related to equations of circles,

## Equations of Circles Worksheet - Problems

Problem 1 :

Write the standard equation of the circle whose center is (-4, 0) and radius is 7.

Problem 2 :

Write the standard equation of the circle whose general equation is

x2 + y- 4x + 6y - 12  =  0

Problem 3 :

The point (1 ,2) is on a circle whose center is (5, -1).  Write the standard equation of the circle.

Problem 4 :

The equation of a circle is

(x + 2)2 + (y - 3)2  =  9

Graph the circle.

Problem 5 :

A bank of lights is arranged over a stage. Each light illuminates a circular area on the stage. A coordinate plane is used to arrange the lights, using the corner of stage as the origin. The equation  (x - 13)2 + (y - 4)2  =  16 represents one of the disks of light.

(i)  Graph the disk of light.

(ii)  Three actors are located as follows :

Henry is at (11, 4)

Jolene is at (8, 5)

Martin is at (15, 5)

Which actors are in the disk of light ?

## Equations of Circles Worksheet - Solutions

Problem 1 :

Write the standard equation of the circle whose center is (-4, 0) and radius is 7.

Solution :

Standard equation of a circle.

(x - h)2 + (y - k)2  =  r2

Plug (h, k)  =  (-4, 0) and r  =  7.

[x - (-4)2] + (y - 0)2  =  72

Simplify.

(x + 4)2 + y2  =  49

Problem 2 :

Write the standard equation of the circle whose general equation is

x2 + y- 4x + 6y - 12  =  0

Solution :

To find the standard equation of the circle, we need to know the center and radius.

Let us find the center and radius from the given general equation of the circle.

Comparing

x2 + y2 + 2gx + 2fy + c  =  0

and

x2 + y2 - 4x + 6y - 12  =  0,

we have

2g  =  - 4 -----> g  =  -2

2f  =  6 -----> f  =  3

c  =  -12

Center  =  (-g, -f)  =  (2, -3)

Radius  =  √[g2 + f2 - c]

Radius  =  √[(-2)2 + 32 - (-12)]

Radius  =  √[4 + 9 + 12]

Standard equation of a circle :

(x - h)2 + (y - k)2  =  r2

Plug (h, k)  =  (2, -3) and r  =  5.

(x - 2)2 + [y - (-3)2]  =  52

Simplify.

(x - 2)2 + (y + 3)2  =  25

Problem 3 :

The point (1 ,2) is on a circle whose center is (5, -1).  Write the standard equation of the circle.

Solution :

To find the standard equation of the circle, we need to know the center and radius. The center is already given and we need to find the radius.

Using distance formula, we have

Radius  =  √[(5 - 1)2 + (-1 - 2)2]

Radius  =  √[42 + (-3)2]

Radius  =  √[16 + 9]

Standard equation of a circle :

(x - h)2 + (y - k)2  =  r2

Plug (h, k)  =  (5, -1) and r  =  5.

(x - 5)2 + [y - (-1)2]  =  52

Simplify.

(x - 5)2 + (y + 1)2  =  52

Problem 4 :

The equation of a circle is

(x + 2)2 + (y - 3)2  =  9

Graph the circle.

Solution :

To graph a circle, we need to know the radius and center of the circle.

Rewrite the equation to find the center and radius.

(x + 2)2 + (y - 3)2  =  9

[x - (-2)2] + (y - 3)2  =  32

So, the center is (-2, 3) and the radius is 3.

To graph the circle, place the point of a compass at (-2, 3), set the radius at 3 units and swing the compass to draw a full circle.

Problem 5 :

A bank of lights is arranged over a stage. Each light illuminates a circular area on the stage. A coordinate plane is used to arrange the lights, using the corner of stage as the origin. The equation  (x - 13)2 + (y - 4)2  =  16 represents one of the disks of light.

(i)  Graph the disk of light.

(ii)  Three actors are located as follows :

Henry is at (11, 4)

Jolene is at (8, 5)

Martin is at (15, 5)

Which actors are in the disk of light ?

Solution :

Solution (i) :

To graph the disk of light, we need to know the center and radius of the circle.

Rewrite the equation to find the center and radius.

(x - 13)2 + (y - 4)2  =  16

(x - 13)2 + (y - 4)2  =  42

So, the center is (13, 4) and the radius is 4.

The circle with center at (13, 4) and radius 4 units shown below (Graph of the disk of light).

Solution (ii) :

The graph given above shows that Henry and Martin are both in the disk of light.

After having gone through the stuff given above, we hope that the students would have understood, "Equations of Circles Worksheet".

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