Geometry (B): READ: Circles Reading

About Circles

Learning Objectives

Distinguish between radius, diameter, chord, tangent, and secant of a circle.
Find relationships between congruent and similar circles.
Examine inscribed and circumscribed polygons.
Write the equation of a circle.

Circle, Center, Radius

A circle is defined as the set of all points that are the same distance away from a specific point called the center of the circle. Note that the circle consists of only the curve but not of the area inside the curve. The distance from the center to the circle is called the radius of the circle.

We often label the center with a capital letter and we refer to the circle by that letter. For example, the circle below is called circle $A$ or $\bigodot A.$

Congruent Circles

Two circles are congruent if they have the same radius, regardless of where their centers are located. For example, all circles of radius of $2\;\mathrm{centimeters}$ are congruent to each other. Similarly, all circles with a radius of $254\;\mathrm{miles}$ are congruent to each other. If circles are not congruent, then they are similar with the similarity ratio given by the ratio of their radii.

Example 1

Determine which circles are congruent and which circles are similar. For similar circles find the similarity ratio.

$\bigodot A$ and $\bigodot D$ are congruent since they both have a radius of $\ 4\;\mathrm{cm}$ .

$\bigodot A$ and $\bigodot B$ are similar with similarity ratio of $\ 1:2$ .

$\bigodot A$ and $\bigodot C$ are similar with similarity ratio of $\ 1:3$ .

$\bigodot B$ and $\bigodot C$ are similar with similarity ratio of $\ 2:3$ .

$\bigodot B$ and $\bigodot D$ are similar with similarity ratio of $\ 2:1$ .

$\bigodot C$ and $\bigodot D$ are similar with similarity ratio of $\ 3:1$ .

Chord, Diameter, Secant

A chord is defined as a line segment starting at one point on the circle and ending at another point on the circle.

A chord that goes through the center of the circle is called the diameter of the circle. Notice that the diameter is twice as long as the radius of the circle.

A secant is a line that cuts through the circle and continues infinitely in both directions.

Point of Tangency and Tangent

A tangent line is defined as a line that touches the circle at exactly one point. This point is called the point of tangency.

Example 2

Identify the following as a secant, chord, diameter, radius, or tangent:

A. $\overline{AC}$

B. $\overline{AB}$

C. $\overline{GH}$

D. $\overline{DE}$

E. $\overline{EF}$

F. $\overline{BC}$

A. $\overline{AC}$ is a diameter of the circle.

B. $\overline{AB}$ is a radius of the circle.

C. $\overline{GH}$ is a chord of the circle.

D. $\overline{DE}$ is a tangent of the circle.

E. $\overline{EF}$ is a secant of the circle.

F. $\overline{BC}$ is a radius of the circle.

Inscribed and Circumscribed Polygons

A convex polygon whose vertices all touch a circle is said to be an inscribed polygon. A convex polygon whose sides all touch a circle is said to be a circumscribed polygon. The figures below show examples of inscribed and circumscribed polygons.

Equations and Graphs of Circles

A circle is defined as the set of all points that are the same distance from a single point called the center. This definition can be used to find an equation of a circle in the coordinate plane.

Let’s consider the circle shown below. As you can see, this circle has its center at point $(2, 2)$ and it has a radius of $\ 3$ .

All the points $x, y$ on the circle are a distance of $3\;\mathrm{units}$ away from the center of the circle.

We can express this information as an equation with the help of the Pythagorean Theorem. The right triangle shown in the figure has legs of length $x - 2$ and $y -2$ and hypotenuse of length $\ 3$ . We write:

$(x-2)^2 + (y-2)^2 = 9$

We can generalize this equation for a circle with center at point $(x_0, y_0)$ and radius $r$ .

$(x-x_0)^2 + (y-y_0)^2 = r^2$

Example 3

Find the center and radius of the following circles:

A. $(x-4)^2 + (y-1)^2 = 25$

B. $(x+1)^2 + (y-2)^2 = 4$

A. We rewrite the equation as: $(x-4)^2 + (y-1)^2 = 5^2.$ The center of the circle is at point $\ (4, 1)$ and the radius is $\ 5$ .

B. We rewrite the equation as: $(x - (-1))^2 + (y - 2)^2 = 2^2.$ The center of the circle is at point $\ (-1, 2)$ and the radius is $\ 2$ .

Example 4

Graph the following circles:

A. $x^2 + y^2 = 9$

B. $(x + 2)^2 + y^2 = 1$

In order to graph a circle, we first graph the center point and then draw points that are the length of the radius away from the center.

A. We rewrite the equation as: $(x-0)^2 + (y-0)^2=3^2.$ The center of the circle is point at $\ (0, 0)$ and the radius is $\ 3$ .

B. We rewrite the equation as: $(x-(-2))^2 + (y-0)^2 = 1^2.$ The center of the circle is point at $\ (-2, 0)$ and the radius is $\ 1$ .

Example 5

Write the equation of the circle in the graph.

From the graph we can see that the center of the circle is at point $\ (-2, 2)$ and the radius is $3\;\mathrm{units}$ long.

Thus the equation is:

$(x + 2)^2 + (y - 2)^2 = 9$

Example 6

Determine if the point $\ (1, 3)$ is on the circle given by the equation: $(x - 1)^2 + (y + 1)^2 = 16.$

In order to find the answer, we simply plug the point $(1, 3)$ into the equation of the circle.

$(1 - 1)^2 + (3 + 1)^2 &= 16\\ 0^2 + 4^2 &= 16$

The point $\ (1, 3)$ satisfies the equation of the circle.

Example 7

Find the equation of the circle whose diameter extends from point $A = (-3, -2)$ to $B = ( 1, 4).$

The general equation of a circle is: $(x - x_0)^2 + (y - y_0)^2 = r^2$

In order to write the equation of the circle in this example, we need to find the center of the circle and the radius of the circle.

Let’s graph the two points on the coordinate plane.

We see that the center of the circle must be in the middle of the diameter.

In other words, the center point is midway between the two points $A$ and $B$ . To get from point $A$ to point $B$ , we must travel $4\;\mathrm{units}$ to the right and $6\;\mathrm{units}$ up. To get halfway from point $A$ to point $B$ , we must travel $2\;\mathrm{units}$ to the right and $3\;\mathrm{units}$ up. This means the center of the circle is at point $(-3 + 2, -2 + 3)$ or $(-1, 1)$ .

We find the length of the radius using the Pythagorean Theorem:

$r^2 = 2^2 + 3^2 \Rightarrow r^2 = 13 \Rightarrow r = \sqrt{13}$

Thus, the equation of the circle is: $(x + 1)^2 + (y - 1)^2 = 13.$

Completing the Square:

You saw that the equation of a circle with center at point $(x_0, y_0)$ and radius $r$ is given by:

$(x - x_0)^2 + (y - y_0)^2 = r^2$

This is called the standard form of the circle equation. The standard form is very useful because it tells us right away what the center and the radius of the circle is.

If the equation of the circle is not in standard form, we use the method of completing the square to rewrite the equation in the standard form.

Example 8

Find the center and radius of the following circle and sketch a graph of the circle.

$x^2 - 4x + y^2 - 6y + 9 = 0$

To find the center and radius of the circle we need to rewrite the equation in standard form. The standard equation has two perfect square factors one for the $x$ terms and one for the $y$ terms. We need to complete the square for the $x$ terms and the $y$ terms separately.

$x^2 - 4x + \underline{\;\;\;\;} + y^2 - 6y + \underline{\;\;\;\;} + 9 = \underline{\;\;\;\;} + \underline{\;\;\;\;}$

To complete the squares we need to find which constants allow us to factors each trinomial into a perfect square. To complete the square for the $x$ terms we need to add a constant of $4$ on both sides.

$x^2 - 4x + \underline{\;4\;} + y^2 - 6y + \underline{\;\;\;\;} + 9 = \underline{\;4\;} + \underline{\;\;\;\;}$

To complete the square for the $y$ terms we need to add a constant of $9$ on both sides.

$x^2 - 4x + \underline{\;4\;} + y^2 - 6y + \underline{\;9\;} + 9 = \underline{\;4\;} + \underline{\;9\;}$

We can factor the separate trinomials and obtain:

$(x - 2)^2 + (y - 3)^2 + 9 = 13$

This simplifies as:

$(x - 2)^2 + (y - 3)^2 = 4$

You can see now that the center of the circle is at point $(2, 3)$ and the radius is $2$ .

Concentric Circles

Concentric circles are circles of different radii that share the same center point.

Example 9

Write the equations of the concentric circles shown in the graph.

$(x - 3)^2 + (y - 2)^2 &= 4\\ (x - 3)^2 + (y - 2)^2 &= 9\\ (x - 3)^2 + (y - 2)^2 &= 16\\ (x - 3)^2 + (y - 2)^2 &= 25$

Example 10

Determine if the circles given by the equations $x^2 - 10x + y^2 - 12y + 57 = 0$ and $x^2 - 10x + y^2 - 12y + 36 = 0$ are concentric.

To find the answer to this question, we must rewrite the equations of the circles in standard form and find the center point of each circle.

To rewrite in standard form, we complete the square on the $x$ and $y$ terms separately.

First circle:

$x^2 - 10x + \underline{\;\;\;\;} + y^2 - 12y + \underline{\;\;\;\;} + 57 & = \underline{\;\;\;\;} + \underline{\;\;\;\;}\\ x^2 - 10x + \underline{\,25\,} + y^2 - 12y + \underline{\,36\,} + 57 & = \underline{\,25\,} + \underline{\,36\,}\\ (x - 5)^2 + (y - 6)^2 + 57 & = 61\\ (x - 5)^2 + (y - 6)^2 & = 4$

The center of the first circle is also at point $(5, 6)$ so the circles are concentric.

Second circle:

$x^2 - 10x + \underline{\;\;\;\;} + y^2 - 12y + \underline{\;\;\;\;} + 36 &= \underline{\;\;\;\;} + \underline{\;\;\;\;}\\ x^2 - 10x + \underline{\,25\,} + y^2 - 12y + \underline{\,36\,} + 36 &= \underline{\,25\,} + \underline{\,36\,}\\ (x - 5)^2 + (y - 6)^2 + 36 &= 61\\ (x - 5)^2 + (y - 6)^2 &= 25$

The center of the second circle is at point $(5, 6)$ .

Lesson Summary

In this section we discussed many terms associated with circles and looked at inscribed and circumscribed polygons. We also covered graphing circles on the coordinate grid and finding the equation of a circle. We found that sometimes we need to use the technique of completing the square to find the equation of a circle.

The following questions are for your own review. The answers are listed below to help you check your work and understanding.

Review Questions

Identify each of the following as a diameter, a chord, a radius, a tangent, or a secant line.
1. $\overline{BG}$
2. $\overline{DG}$
3. $\overline{DC}$
4. $\overline{FE}$
5. $\overline{AG}$
6. $\overline{DB}$
Determine which of the following circles are congruent and which are similar. For circles that are similar give the similarity ratio.

For exercises 3-8, find the center and the radius of the circles:

$x^2 + y^2 = 1$
$(x - 3)^2 + (y + 5)^2 = 81$
$x^2 + (y - 2)^2 = 4$
$(x + 6)^2 + (y + 1)^2 =25$
Check that the point $\ (3, 4)$ is on the circle given by the equation $x^2 + (y - 4)^2 = 9.$
Check that the point $\ (-5, 5)$ is on the circle given by the equation $(x + 3)^2 + (y + 2)^2 = 50.$
Write the equation of the circle with center at $\ (2, 0)$ and radius $\ 4$ .
Write the equation of the circle with center at $\ (4, 5)$ and radius $\ 9$ .
Write the equation of the circle with center at $\ (-1, -5)$ and radius $\ 10$ .

For 14 and 15, write the equation of the circles.

In a circle with center $\ (4, 1)$ one endpoint of a diameter is $\ (-1, 3)$ . Find the other endpoint of the diameter.
The endpoints of the diameter of a circle are given by the points $A = (1, 4)$ and $B = (7, 2)$ . Find the equation of the circle.
A circle has center $(0, 4)$ and contains point $(1, 1)$ . Find the equation of the circle.
A circle has center $(-2, -2)$ and contains point $(4, 4)$ . Find the equation of the circle.
Find the center and the radius of the following circle: $x^2 + 8x + y^2 - 2y - 19 = 0$ .
Find the center and the radius of the following circle: $x^2 - 10x + y^2 + 6y -15 =0$ .
Find the center and the radius of the following circle: $x^2 + 20x + y^2 - 30y + 181 = 0$ .
Determine if the circles given by the equations are concentric.
1. $x^2 - 4x + y^2 + 2y + 4 = 0$ and $x^2 - 4x + y^2 + 2y - 31 = 0$
2. $x^2 + 6x + y^2 + 8y = 0$ and $x^2 + 6x + y^2 - 8y + 9 = 0$
3. $x^2 + y^2 + 21 = 10y$ and $x^2 + y^2 - 8y = 33$
4. $x^2 + y^2 + 26 = 10x + 10y$ and $x^2 + y^2 = 10x + 10y + 14$

Review Answers

1. $\overline{BG}$ is a radius.
2. $\overline{DG}$ is a diameter.
3. $\overline{DC}$ is a tangent.
4. $\overline{FE}$ is a secant.
5. $\overline{AG}$ is a chord.
6. $\overline{DB}$ is a radius.
$\bigodot L$ is congruent to $\bigodot M$ ; $\bigodot L$ is similar to $\bigodot N$ with similarity ratio $2:7; \bigodot M$ is similar to $\bigodot N$ with similarity ratio $2:7$ .
The center is located at $(0, 0), \mathrm{radius} = 4$ .
The center is located at $(-1, -2), \mathrm{radius} = 2$ .
The center is located at $(0, 0), \mathrm{radius} = 1$ .
The center is located at $(3, -5), \mathrm{radius} = 9$ .
The center is located at $(0, 2), \mathrm{radius} = 2$ .
The center is located at $(-6, -1), \mathrm{radius} = 5$ .
$3^2 + (4 - 4)^2 = 3^2 = 9.$ The point is on the circle.
$(-5 + 3)^2 + (5 + 2)^2 = 4 + 49 = 53 \neq 50.$ The point is not on the circle.
$(x - 2)^2 + y^2 = 16$
$(x - 4)^2 + (y - 5)^2 = 81$
$(x + 1)^2 + (y + 5)^2 = 100$
$(x - 1)^2 + (y - 2)^2 = 4$
$x^2 + (y + 1)^2 = 9$
$(9, -1)$
$(x - 4)^2 + (y - 3)^2 = 10$
$x^2 + (y - 4)^2 = 10$
$(x + 2)^2 + (y + 2)^2 = 72$
The center is located at $(-4, 1), \mathrm{radius} = 6$ .
The center is located at $(5, -3), \mathrm{radius} = 7$ .
The center is located at $(-10, 15), \mathrm{radius} = 12$ .
1. Yes
2. No
3. No
4. Yes

Last modified: Tuesday, June 29, 2010, 11:14 AM