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# 9.1: Parts of Circles

Difficulty Level: At Grade Created by: CK-12
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Practice Parts of Circles

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What if you were asked to geometrically consider the ancient astronomical clock in Prague, pictured below? It has a large background circle that tells the local time and the “ancient time” and then the smaller circle rotates around on the orange line to show the current astrological sign. The yellow point is the center of the larger clock. How does the orange line relate to the small and larger circle? How does the hand with the moon on it (black hand with the circle) relate to both circles? Are the circles concentric or tangent?

### Parts of Circles

A circle is the set of all points in the plane that are the same distance away from a specific point, called the center. The center of the circle below is point A\begin{align*}A\end{align*}. We call this circle “circle A\begin{align*}A\end{align*},” and it is labeled A\begin{align*}\bigodot A\end{align*}.

##### Important Circle Parts

Radius: The distance from the center of the circle to its outer rim.

Chord: A line segment whose endpoints are on a circle.

Diameter: A chord that passes through the center of the circle. The length of a diameter is two times the length of a radius.

Secant: A line that intersects a circle in two points.

Tangent: A line that intersects a circle in exactly one point.

Point of Tangency: The point where a tangent line touches the circle.

The tangent ray TP\begin{align*}\overrightarrow{TP}\end{align*} and tangent segment TP¯¯¯¯¯¯¯\begin{align*}\overline{TP}\end{align*} are also called tangents.

Tangent Circles: Two or more circles that intersect at one point.

Two circles can be tangent to each other in two different ways, either internally tangent or externally tangent.

If the circles are not tangent, they can share a tangent line, called a common tangent. Common tangents can be internally tangent and externally tangent too. Notice that the common internal tangent passes through the space between the two circles. Common external tangents stay on the top or bottom of both circles.

Concentric Circles: Two or more circles that have the same center, but different radii.

Congruent Circles: Two or more circles with the same radius, but different centers.

Watch the first half of this video.

#### Identifying Parts of Circles

Find the parts of A\begin{align*}\bigodot A\end{align*} that best fit each description.

HA¯¯¯¯¯¯¯¯\begin{align*}\overline{HA}\end{align*} or AF¯¯¯¯¯¯¯¯\begin{align*}\overline{AF}\end{align*}

b) A chord

CD¯¯¯¯¯¯¯¯, HF¯¯¯¯¯¯¯¯\begin{align*}\overline{CD}, \ \overline{HF}\end{align*}, or DG¯¯¯¯¯¯¯¯\begin{align*}\overline {DG}\end{align*}

c) A tangent line

BJ\begin{align*}\overleftrightarrow{BJ}\end{align*}

d) A point of tangency

Point H

e) A diameter

HF¯¯¯¯¯¯¯¯\begin{align*}\overline{HF}\end{align*}

f) A secant

BD\begin{align*}\overleftrightarrow{BD}\end{align*}

#### Drawing Intersecting Cricles

Draw an example of how two circles can intersect with no, one and two points of intersection. You will make three separate drawings.

#### Determining if Circles are Congruent

Determine if any of the following circles are congruent.

From each center, count the units to the outer rim of the circle. It is easiest to count vertically or horizontally. Doing this, we have:

Radius of ARadius of BRadius of C=3 units=4 units=3 units\begin{align*}\text{Radius of} \ \bigodot A & = 3 \ units\\ \text{Radius of} \ \bigodot B & = 4 \ units\\ \text{Radius of} \ \bigodot C & = 3 \ units\end{align*}

From these measurements, we see that AC\begin{align*}\bigodot A \cong \bigodot C\end{align*}.

Notice the circles are congruent. The lengths of the radii are equal.

Refer to the photograph at the beginning of this section. The orange line (which is normally black, but outlined for the purpose of this exercise) is a diameter of the smaller circle. Since this line passes through the center of the larger circle (yellow point, also outlined), it is part of one of its diameters. The “moon” hand is a diameter of the larger circle, but a secant of the smaller circle. The circles are not concentric because they do not have the same center and are not tangent because the sides of the circles do not touch.

### Examples

#### Example 1

If the diameter of a circle is 10 inches, how long is the radius?

The radius is always half the length of the diameter, so it is 5 inches.

#### Example 2

Is it possible to have a line that intersects a circle three times? If so, draw one. If not, explain.

It is not possible. By definition, all lines are straight. The maximum number of times a line can intersect a circle is twice

#### Example 3

Are all circles similar?

Yes. All circles are the same shape, but not necessarily the same size, so they are similar.

### Review

Determine which term best describes each of the following parts of P\begin{align*}\bigodot P\end{align*}.

1. KG¯¯¯¯¯¯¯¯\begin{align*}\overline{KG}\end{align*}
2. FH\begin{align*}\overleftrightarrow{FH}\end{align*}
3. KH¯¯¯¯¯¯¯¯¯\begin{align*}\overline{KH}\end{align*}
4. E\begin{align*}E\end{align*}
5. BK\begin{align*}\overleftrightarrow{BK}\end{align*}
6. CF\begin{align*}\overleftrightarrow{CF}\end{align*}
7. A\begin{align*}A\end{align*}
8. JG¯¯¯¯¯¯¯\begin{align*}\overline{JG}\end{align*}
9. HG¯¯¯¯¯¯¯¯\begin{align*}\overline{HG}\end{align*}
10. What is the longest chord in any circle?

Use the graph below to answer the following questions.

1. Find the radius of each circle.
2. Are any circles congruent? How do you know?
3. How are C\begin{align*}\bigodot C\end{align*} and E\begin{align*}\bigodot E\end{align*} related?
4. Find the equation of CE\begin{align*}\overleftrightarrow{CE}\end{align*}.
5. Find the length of CE¯¯¯¯¯¯¯¯\begin{align*}\overline{CE}\end{align*}.

### Notes/Highlights Having trouble? Report an issue.

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### Vocabulary Language: English Spanish

TermDefinition
chord A line segment whose endpoints are on a circle.
concentric circles Two or more circles that have the same center, but different radii.
congruent circles Two or more circles with the same radius, but different centers.
Diameter Diameter is the measure of the distance across the center of a circle. The diameter is equal to twice the measure of the radius.
point of tangency The point where the tangent line touches the circle.
Tangent Circles Tangent Circles are two or more circles that intersect at one point.
Circle A circle is the set of all points at a specific distance from a given point in two dimensions.
Circumference The circumference of a circle is the measure of the distance around the outside edge of a circle.
Radius The radius of a circle is the distance from the center of the circle to the edge of the circle.
Secant The secant of an angle in a right triangle is the value found by dividing length of the hypotenuse by the length of the side adjacent the given angle. The secant ratio is the reciprocal of the cosine ratio.
Tangent The tangent of an angle in a right triangle is a value found by dividing the length of the side opposite the given angle by the length of the side adjacent to the given angle.

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