{{ toc.signature }}
{{ toc.name }}
{{ stepNode.name }}
Proceed to next lesson
An error ocurred, try again later!
Chapter {{ article.chapter.number }}
{{ article.number }}.

# {{ article.displayTitle }}

{{ article.introSlideInfo.summary }}
{{ 'ml-btn-show-less' | message }} {{ 'ml-btn-show-more' | message }} expand_more
##### {{ 'ml-heading-abilities-covered' | message }}
{{ ability.description }}

#### {{ 'ml-heading-lesson-settings' | message }}

{{ 'ml-lesson-show-solutions' | message }}
{{ 'ml-lesson-show-hints' | message }}
 {{ 'ml-lesson-number-slides' | message : article.introSlideInfo.bblockCount}} {{ 'ml-lesson-number-exercises' | message : article.introSlideInfo.exerciseCount}} {{ 'ml-lesson-time-estimation' | message }}
In this lesson, some theorems about lines and angles will be explored and proven. The theorems will be applied using real-life examples.

### Catch-Up and Review

Here are a few recommended readings before getting started with this lesson.

## Investigating Properties of Vertical Angles

Before any theorems will be introduced, try to discover some properties of angles using the interactive applet. While exploring, think about how those properties could be proven. The applet allows for translations and rotations of the angles. Consider a pair of vertical angles. ### Extra

How to Use the Applet
• To translate an angle, click on the sides of the angle or in the region between the sides.
• To rotate an angle about move the corresponding slider.

## Congruence of Vertical Angles

Vertical angles can be mapped onto each other by using a rotation. Since rotations are rigid motions, the angle measures are preserved. This leads to the following theorem.

## Vertical Angles Theorem

Vertical angles are always congruent. Based on the characteristics of the diagram, the following relations hold true.

### Proof

Geometric Approach

Analyzing the diagram, it can be seen that and form a straight angle, so these are supplementary angles. Similarly, and are also supplementary angles. Therefore, by the Angle Addition Postulate, the sum of and is and the sum of and is also These facts can be used to express in terms of and in terms of

By the Transitive Property of Equality, the expressions representing can be set equal to each other.
Then the equation can be simplified. By the definition of congruent angles, this means that the vertical angles and are congruent angles. The same process can be used to prove and congruent.

### Two-Column Proof

The previous proof can be summarized in the following two-column table.

 Statements Reasons and lines Given and supplementary Definition of straight angle Definition of supplementary angles Subtraction Property of Equality and supplementary Definition of straight angle Definition of supplementary angles Subtraction Property of Equality Transitive Property of Equality Subtraction and Multiplication Properties of Equality

### Proof

Using Transformations

Consider the points and on each ray that starts at the point of intersection of the two lines. Suppose that points and are rotated about point The points and are mapped onto the points and after the rotation. This means that is mapped onto Since rotations are a rigid motion, and are congruent angles.
Since the point lies on and point lies on is congruent to
By applying the Transitive Property of Congruence, it can be confirmed that is congruent to

## Solving Problems Using Vertical Angles

In Flowerland Village, there is a crossroad between Tulip Street and Rose Street. There is a plan to continue the construction of Tulip Street toward the southwest. At the moment, the crossroad forms two angles, whose measures are expressed by and respectively. Find the measures of all four angles the crossroad will form after the construction of Tulip Street is finished.

### Hint

Use the given expressions to form an equation for Identify the relationship between and as well as and by analyzing their positions.

### Solution

The angles formed by the crossroad before the construction form a linear pair. Therefore, they are supplementary angles. Using this information, the following equation can be formed.
By solving it, the value of can be determined.
Solve for
Now that the value of is known, the measures of and can be calculated.
Next, by analyzing the position of and as well as and it can be noted that these are vertical angles. Therefore, by the Vertical Angles Theorem, they are two pairs of congruent angles.
In this way, it was obtained that and are each and and are each ## Investigating Parallel Lines Cut by a Transversal

Consider two parallel lines cut by a transversal. The applet shows a pair of corresponding angles, and Is it possible to translate one line so that one of these angles maps onto another? ## Corresponding Angles Theorem and Its Converse

The observed relation between corresponding angles is presented and proven in the following theorem.

## Corresponding Angles Theorem

If parallel lines are cut by a transversal, then the pairs of corresponding angles are congruent. Based on the characteristics of the diagram, the following relations hold true.

If then and

### Proof

It is given that and are parallel lines. Therefore, by the definition of parallel lines, there is a translation that maps one line onto another. It should be noted that when translating one line onto another, the pairs of corresponding angles overlap and seem to have the same measures. In fact, because a translation is a rigid motion, and a rigid motion preserves length and angle measures, the pairs of corresponding angles are congruent angles.

Note that the converse statement is also true.

## Converse Corresponding Angles Theorem

If two lines and a transversal form corresponding angles that are congruent, then the lines are parallel. Based on the characteristics of the diagram, the following relation holds true.

If or then

### Proof

This theorem can be proven by an indirect proof. Let and be two lines intersected by a transversal line forming corresponding congruent angles and Since the goal is to prove that is parallel to , it will be temporarily assumed that and are not parallel.
By the Parallel Postulate, there exists a line parallel to that passes through the point of intersection between and This line forms and By the Angle Addition Postulate, is equal to the sum of and
Since and are parallel lines that are cut by a transversal, by the Corresponding Angles Theorem, and are congruent. By the definition of congruence, these angles have the same measure.
By the Substitution Property of Equality, can be substituted for into the equation for
From the above equation and since is a positive number, it can be concluded that is greater than
This contradicts the given fact that and are congruent. The contradiction came from assuming that and are not parallel lines. Therefore, and must be parallel lines.

## Solving Problems Using Corresponding Angles

In a Flowerland Village house, there are stairs with hand railings like shown in the diagram. The measures of and are expressed as and respectively. What are the measures of and

### Hint

How do measures of and relate to each other? Use the given expressions to form an equation for

### Solution

Analyzing the diagram, it can be noted that and are corresponding angles formed by two parallel lines and a transversal. Therefore, by the Corresponding Angles Theorem, these angles are congruent. Hence, the measures of these angles are the same.
By substituting with and with the equation for can be formed.
Solve for
Now that the value of is known, the measure of each of the angles can be calculated.
Substitute for and evaluate
Since the angles are congruent, it can be concluded that they both measure to be

## Alternate Interior Angles Theorem and Its Converse

Like corresponding angles, alternate interior angles are also formed by two parallel lines cut by a transversal.

## Alternate Interior Angles Theorem

If parallel lines are cut by a transversal, then alternate interior angles are congruent. Based on the characteristics of the diagram, the following relations hold true.

If then and

### Proof

Geometric approach

To prove that alternate interior angles are congruent, it will be shown that and are congruent. Notice that by definition and are vertical angles. By the Vertical Angles Theorem, they are therefore congruent angles.
Furthermore, by definition and are corresponding angles. Hence, by the Corresponding Angles Theorem, and are also congruent angles.
Applying the Transitive Property of Congruence, and can be concluded to be congruent angles as well.
The same reasoning applies to the other pair of alternate interior angles. Therefore, when a pair of parallel lines is cut by a transversal, the pairs of alternate interior angles are congruent.

### Two-Column Proof

The previous proof can be summarized in the following two-column table.

 Statements Reasons and are vertical angles Def. of vertical angles Vertical Angles Theorem and are corresponding angles Def. of corresponding angles Corresponding Angles Theorem Transitive Property of Congruence

### Proof

Using Transformations

Apart from the points of intersection, consider two more points on each line. Next, and will be translated parallel to the transversal until the points and lie on Then, and will be rotated about It should be noted that since point lies on the transversal, when translating it to the point will fall into the same position as Therefore, will not be affected by the rotation around After this combination of rigid motions, and are mapped onto and This means that is mapped onto Therefore, and are congruent angles.
Note that and share the same location. It can also be seen that lies on and lies on Because of this, is congruent to
Applying the Transitive Property of Congruence, is congruent to

The converse statement is also true.

## Converse Alternate Interior Angles Theorem

If two lines and a transversal form alternate interior angles that are congruent, then the two lines are parallel. Based on the characteristics of the diagram, the following relation holds true.

If or then

### Proof

The proof will be based on the given diagram, but it holds true for any pair of lines cut by a transversal. Consider only one pair of congruent alternate interior angles and one more angle. It needs to be proven that and are parallel lines. It is already given that is congruent to
The diagram shows that and are vertical angles. By the Vertical Angles Theorem, these angles are congruent.
Notice the common angle of in both relationships. By the Transitive Property of Congruence, since is congruent to and is congruent to then is congruent
The diagram also shows that and are corresponding angles. Given that relation, the Converse Corresponding Angles Theorem can be applied.
 Converse Corresponding Angles Theorem If two lines are cut by a transversal so that the corresponding angles are congruent, then the lines are parallel.

Since and are corresponding congruent angles, then and are parallel lines. To summarize, all of the steps will be described in a two-column proof.

 Statement Reason Given Vertical Angles Theorem Transitive Property of Congruence Converse Corresponding Angles Theorem

## Alternate Exterior Angles Theorem and Its Converse

Similar properties can be discovered for alternate exterior angles.

## Alternate Exterior Angles Theorem

If parallel lines are cut by a transversal, then the pairs of alternate exterior angles are congruent. Based on the characteristics of the diagram, the following relations hold true.

If then and

### Proof

Geometric Approach

In order to prove that alternate exterior angles are congruent, it will be shown that and are congruent. Notice that by definition, and are corresponding angles. Therefore, by the Corresponding Angles Theorem, they are congruent angles.
Furthermore, by definition, and are vertical angles. Therefore, by the Vertical Angles Theorem, and are congruent angles.
Then, by applying the Transitive Property of Congruence, and can be concluded to be congruent angles as well.
The same reasoning applies to the other pair of alternate exterior angles. Therefore, when a pair of parallel lines is cut by a transversal, the pairs of alternate exterior angles are congruent.

### Two-Column Proof

The previous proof can be summarized in the following two-column table.

 Statements Reasons and are corresponding angles Def. of corresponding angles Corresponding Angles Theorem and are vertical angles Def. of vertical angles Vertical Angles Theorem Transitive Property of Congruence

### Proof

Using Transformations

Consider the points of intersection as well as two more points on each line. Next, and will be translated in the direction of the transversal so that points and lie on Then, and will be rotated about After this combination of rigid motions, and are mapped onto and This means that is mapped onto Therefore, and are congruent angles.
Since and share the same location, lies on and lies on Because of this, is congruent to
Applying the Transitive Property of Congruence, is congruent to
It has been proved that one pair of alternate exterior angles is congruent. Further, since lies on it can also be proven that is congruent to
Applying the Transitive Property of Congruence again, is congruent to
To conclude, it has been obtained that both pairs of alternate exterior angles are congruent.

and

## Converse Alternate Exterior Angles Theorem

If two lines and a transversal form alternate exterior angles that are congruent, then the two lines are parallel. Based on the properties of the diagram, the following relation holds true.

If or then

### Proof

The proof will be based on the given diagram, but it holds true for any pair of lines cut by a transversal. Consider only one pair of congruent alternate exterior angles and one more angle. It needs to be proven that and are parallel lines. It is already given that is congruent to
From the diagram, it can also be noted that and are vertical angles. By the Vertical Angles Theorem, these angles are congruent.
By the Transitive Property of Congruence, because is congruent to and is congruent to is congruent to
Further, and are corresponding angles. Hence, the Converse Corresponding Angles Theorem can be applied.
 Converse Corresponding Angles Theorem If two lines are cut by a transversal so that the corresponding angles are congruent, then the lines are parallel.

Since and are corresponding congruent angles, and are parallel lines. Each step of the proof will now be summarized in a two-column proof.

 Statement Reason Given Vertical Angles Theorem Transitive Property of Congruence Converse Corresponding Angles Theorem

## Using Alternate Interior Angles to Solve Problems

In order to build Tulip Street on the south side of the Lilian river, which goes through Flowerland Village, there is a need to build a bridge. Devontay, an architect, proposed the following plan for the bridge. It is known that the measure of is equal to and the measure of is equal to What are the measures of and

### Hint

How do the measures of and relate to each other? Use the given expressions to form an equation for

### Solution

By analyzing the diagram it can be noted that and are alternate interior angles.