Donyell walks from his house to his school. Then, after school he walks to his friend's house. He notices that his journey forms an angle. He decides to name the angle. Donyell labels his house as point \begin{align*}H\end{align*} , his school as point \begin{align*}S\end{align*} and his friend's house as point \begin{align*}F\end{align*}. Can you use the points to name of the angle formed by his journey?

In this concept, you will learn about the parts of an angle.

### Identifying Angles by Vertex and Ray

An **angle** is one of the key geometric figures that you will be working with in geometry. An angle is created when two rays connect at a common point.

You can see that the two rays are connected at a common endpoint, called a **vertex**. This forms the angle. An angle is named by points on the rays.

Let's look at an example of an angle.

This is angle \begin{align*}ABC\end{align*}. The vertex \begin{align*}B\end{align*} is always in the middle. The symbol for angle looks like a small angle.

\begin{align*}\angle{ABC}\end{align*}

Angle \begin{align*}ABC\end{align*} is named with this symbol.

### Examples

#### Example 1

Earlier, you were given a problem about Donyell and his walk to school and then to his friend's house.

Name the angle if his house is point \begin{align*}H\end{align*}, his school is point \begin{align*}S\end{align*} and his friend's house is point \begin{align*}F\end{align*}.

First, determine the common endpoint.

\begin{align*}S\end{align*}

Then, name the angle.

\begin{align*}\angle HSF\end{align*}

The answer is \begin{align*}\angle HSF\end{align*}.

#### Example 2

What is the common endpoint of the rays that form \begin{align*}\angle HIJ\end{align*}?

First, determine the name of the common endpoint.

The vertex

Next, identify the location of the vertex in the angle name.

The middle point

Then, write the point.

\begin{align*}I\end{align*}

The answer is \begin{align*}I\end{align*}. The common endpoint of the rays that form \begin{align*}\angle HIJ\end{align*} is \begin{align*}I\end{align*}.

#### Example 3

Name this angle. Be sure that the vertex is in the middle.

First, locate the common endpoint.

\begin{align*}E\end{align*}

Then, name the angle.

\begin{align*}\angle DEF\end{align*}

The answer is** \begin{align*}\angle{DEF}\end{align*}**.

#### Example 4

First, locate the common endpoint.

\begin{align*}M\end{align*}

Then, name the angle.

\begin{align*}\angle LMN\end{align*}

The answer is **\begin{align*}\angle{LMN}\end{align*}**

#### Example 5

What is the common endpoint of the rays that form \begin{align*}\angle QRS\end{align*}?

First, determine the name of the common endpoint.

The vertex

Next, identify the location of the vertex in the angle name.

The middle point

Then, write the point.

\begin{align*}R\end{align*}

The answer is \begin{align*}R\end{align*}. The common endpoint of the rays that form \begin{align*}\angle QRS\end{align*} is \begin{align*}R\end{align*}.

### Review

Draw a picture to illustrate each of the named geometric figures.

- \begin{align*}\overrightarrow{AB}\end{align*}
- \begin{align*}\overleftrightarrow{CD}\end{align*}
- \begin{align*}\overleftrightarrow{DE}\end{align*}
- \begin{align*}\angle{ABC}\end{align*}
- \begin{align*}\angle{LMN}\end{align*}
- \begin{align*}\overline{XY}\end{align*}
- \begin{align*}\overrightarrow{PQ}\end{align*}
- \begin{align*}\overleftrightarrow{GH}\end{align*}
- \begin{align*}\overleftrightarrow{AB} \parallel \overleftrightarrow{DE}\end{align*}
- \begin{align*}\overleftrightarrow{LM} \parallel \overleftrightarrow{DE}\end{align*}
- \begin{align*}\overleftrightarrow{RS} \parallel \overleftrightarrow{TU}\end{align*}
- \begin{align*}\overline{DF} \parallel \overline{XY}\end{align*}

Define the following terms.

- Ray
- Line
- Vertex
- Line Segment

### Review (Answers)

To see the Review answers, open this PDF file and look for section 9.3.