What if you were presented with two angles that are on opposite sides of a transversal, but inside the lines? How would you describe these angles and what could you conclude about their measures? After completing this Concept, you'll be able to answer these questions using your knowledge of alternate interior angles.

### Watch This

CK-12 Foundation: Chapter3AlternateInteriorAnglesA

Watch the portions of this video dealing with alternate interior angles.

James Sousa: Angles and Transversals

James Sousa: Proof that Alternate Interior Angles Are Congruent

James Sousa: Proof of Alternate Interior Angles Converse

### Guidance

**Alternate Interior Angles** are two angles that are on the interior of and , but on opposite sides of the transversal. * and are alternate interior angles.*

**Alternate Interior Angles Theorem:** If two parallel lines are cut by a transversal, then the alternate interior angles are congruent.

**Proof of Alternate Interior Angles Theorem:**

Given:

Prove:

Statement |
Reason |
---|---|

1. | Given |

2. | Corresponding Angles Postulate |

3. | Vertical Angles Theorem |

4. | Transitive PoC |

There are several ways we could have done this proof. For example, Step 2 could have been for the same reason, followed by . We could have also proved that .

**Converse of Alternate Interior Angles Theorem:** If two lines are cut by a transversal and alternate interior angles are congruent, then the lines are parallel.

#### Example A

Find .

because they are corresponding angles and the lines are parallel. and are vertical angles, so also.

and the angle are alternate interior angles.

#### Example B

Find the measure of the angle and .

The two given angles are alternate interior angles so, they are equal. Set the two expressions equal to each other and solve for .

#### Example C

Prove the Converse of the Alternate Interior Angles Theorem.

Given: and and transversal

Prove:

Statement |
Reason |
---|---|

1. and and transversal | Given |

2. | Vertical Angles Theorem |

3. | Transitive PoC |

4. | Converse of the Corresponding Angles Postulate |

Watch this video for help with the Examples above.

CK-12 Foundation: Chapter3AlternateInteriorAnglesB

### Vocabulary

** Alternate Interior Angles** are two angles that are on the interior of and , but on opposite sides of the transversal.

### Guided Practice

1. Is ?

2. What does have to be to make ?

3. List the pairs of alternate interior angles:

**Answers:**

1. First, find . We know its linear pair is . By the Linear Pair Postulate, these two angles add up to , so . This means that , by the Converse of the Corresponding Angles Postulate.

2. Because these are alternate interior angles, they must be equal for . Set the expressions equal to each other and solve.

3. Alternate Interior Angles: and , and .

### Interactive Practice

### Practice

- Is the angle pair and congruent, supplementary or neither?
- Give two examples of alternate interior angles in the diagram:

For 3-4, find the values of .

For question 5, use the picture below. Find the value of .

- Are lines and parallel? If yes, how do you know?

For 7-12, what does the value of have to be to make the lines parallel?

- and
- and
- and
- and
- and
- and

For questions 13-15, use the picture below.

- What is the alternate interior angle to ?
- What is the alternate interior angle to ?
- Are the two lines parallel? Explain.