What if you want to know how opposite pairs of angles are related when two lines cross, forming four angles? After completing this Concept, you'll be able to apply the properties of these special angles to help you solve problems in geometry.

### Watch This

CK-12 Foundation: Chapter1VerticalAnglesA

### Guidance

**Vertical angles** are two non-adjacent angles formed by intersecting lines. In the picture below, and are vertical angles and and are vertical angles.

Notice that these angles are labeled with numbers. You can tell that these are labels because they do not have a degree symbol.

##### Investigation: Vertical Angle Relationships

- Draw two intersecting lines on your paper. Label the four angles created and . See the picture above.
- Take your protractor and find .
- What is the angle relationship between and ? Find .
- What is the angle relationship between and ? Find .
- What is the angle relationship between and ? Find .
- Are any angles congruent? If so, write down the congruence statement.

From this investigation, hopefully you found out that and . This is our first theorem. That means it must be proven true in order to use it.

**Vertical Angles Theorem:** If two angles are vertical angles, then they are congruent.

We can prove the Vertical Angles Theorem using the same process we used above. However, let’s not use any specific values for the angles.

Recall that anytime the measures of two angles are equal, the angles are also congruent.

#### Example A

Find and .

is vertical angles with , so . is a linear pair with or , so .

#### Example B

Name one pair of vertical angles in the diagram below.

One example is and .

#### Example C

If and are vertical angles and and , what is the measure of each angle?

Vertical angles are congruent, so set the angles equal to each other and solve for . Then go back to find the measure of each angle.

So,

Watch this video for help with the Examples above.

CK-12 Foundation: Chapter1VerticalAnglesB

### Vocabulary

** Vertical angles** are two non-adjacent angles formed by intersecting lines. They are always congruent.

### Guided Practice

Find the value of or .

1.

2.

3.

**Answers:**

1. Vertical angles are congruent, so set the angles equal to each other and solve for .

2. Vertical angles are congruent, so set the angles equal to each other and solve for .

3. Vertical angles are congruent, so set the angles equal to each other and solve for .

### Interactive Practice

### Practice

Use the diagram below for exercises 1-2. Note that .

- Name one pair of vertical angles.

- If , find .

For exercises 3-5, determine if the statement is true or false.

- Vertical angles have the same vertex.
- Vertical angles are supplementary.
- Intersecting lines form two pairs of vertical angles.

Solve for the variables and for exercises 6-10.

- If and are vertical angles and and , what is the measure of each angle?
- If and are vertical angles and and , what is the measure of each angle?
- If and are vertical angles and and , what is the measure of each angle?
- If and are vertical angles and and , what is the measure of each angle?