# Eureka Math Geometry Module 1 Lesson 10 Answer Key

## Engage NY Eureka Math Geometry Module 1 Lesson 10 Answer Key

### Eureka Math Geometry Module 1 Lesson 10 Example Answer Key

Examples

Example 1.
In the figure to the right, $$\overline{A B}$$ || $$\overline{C D}$$ and $$\overline{B C}$$ || $$\overline{D E}$$. Prove that mâˆ ABC=mâˆ CDE.
(Is an auxiliary segment necessary?)

$$\overline{A B}$$ || $$\overline{C D}$$, $$\overline{B C}$$ || $$\overline{D E}$$ Given
mâˆ ABC = mâˆ BCD If parallel lines are cut by a transversal, then alternate interior angles are equal in measure.
mâˆ BCD = mâˆ CDE If parallel lines are cut by a transversal, then alternate interior angles are equal in measure.
mâˆ ABC = mâˆ CDE Transitive property

Example 2.
In the figure to the right, $$\overline{A B}$$ || $$\overline{C D}$$ and $$\overline{B C}$$ || $$\overline{D E}$$. Prove that b+d=180.

Label cÂ°.
$$\overline{A B}$$ || $$\overline{C D}$$, $$\overline{B C}$$ || $$\overline{D E}$$ Given
b=c If parallel lines are cut by a transversal, then alternate interior angles are equal in measure.
c+d=180 If parallel lines are cut by a transversal, then same-side interior angles are supplementary.
b+d=180 Substitution property of equality

Example 3.
In the figure to the right, prove that d=a+b+c.
Label Z and z.
z=b+c The exterior angle of a triangle equals the sum of the two interior opposite angles.
d=z+a The exterior angle of a triangle equals the sum of the two interior opposite angles.
d=a+b+c Substitution property of equality

### Eureka Math Geometry Module 1 Lesson 10 Exercise Answer Key

Opening Exercise
In the figure to the right, $$\overline{A B}$$ || $$\overline{D E}$$ and $$\overline{B C}$$ || $$\overline{E F}$$. Prove that b=e.
(Hint: Extend $$\overline{B C}$$ and $$\overline{E D}$$.)

PROOF:
$$\overline{A B}$$ || $$\overline{D E}$$, $$\overline{B C}$$ || $$\overline{E F}$$ Given
b=z If parallel lines are cut by a transversal, then alternate interior angles are equal in measure.
z=e If parallel lines are cut by a transversal, then alternate interior angles are equal in measure.
b=e Transitive property

In the previous lesson, you used deductive reasoning with labeled diagrams to prove specific conjectures. What is different about the proof above?

Drawing or extending segments, lines, or rays (referred to as auxiliary lines) is frequently useful in demonstrating steps in the deductive reasoning process. Once $$\overline{B C}$$ and $$\overline{E D}$$ were extended, it was relatively simple to prove the two angles congruent based on our knowledge of alternate interior angles. Sometimes there are several possible extensions or additional lines that would work equally well.
For example, in this diagram, there are at least two possibilities for auxiliary lines. Can you spot them both?

Given: $$\overline{A B}$$ || $$\overline{C D}$$.
Prove: z=x+y.

Discussion
Here is one possibility:
Given: $$\overline{A B}$$ || $$\overline{C D}$$.
Prove: z = x + y.
Extend the transversal as shown by the dotted line in the diagram. Label angle measures v and w, as shown.
What do you know about v and x?

Write a proof using the auxiliary segment drawn in the diagram to the right.
$$\overline{A B}$$ || $$\overline{C D}$$ Given.
z=v+w The exterior angle of a triangle equals the sum of the two interior opposite angles.
x=v If parallel lines are cut by a transversal, then corresponding angles are equal in measure.
y=w If parallel lines are cut by a transversal, then corresponding angles are equal in measure.
z=x+y Substitution property of equality

Another possibility appears here:

Given: $$\overline{A B}$$ || $$\overline{C D}$$.
Prove: z=x+y.
Draw a segment parallel to $$\overline{A B}$$ through the vertex of the angle measuring z degrees. This divides the angle into two parts as shown.

What do you know about v and x?
They are equal since they are corresponding angles of parallel lines crossed by a transversal.

They are also equal in measure since they are corresponding angles of parallel lines crossed by a transversal.

Write a proof using the auxiliary segment drawn in this diagram. Notice how this proof differs from the one above.
$$\overline{A B}$$ || $$\overline{C D}$$ Given
x=v If parallel lines are cut by a transversal, the corresponding angles are equal.
y=w If parallel lines are cut by a transversal, the corresponding angles are equal.
z=x+y Substitution property of equality

### Eureka Math Geometry Module 1 Lesson 10 Problem Set Answer Key

Question 1.
In the figure to the right, $$\overline{A B}$$ || $$\overline{D E}$$ and $$\overline{B C}$$ || $$\overline{E F}$$. Prove that mâˆ ABC=mâˆ DEF.

Extend $$\overline{D E}$$ through $$\overline{B C}$$, and mark the intersection with $$\overline{B C}$$ as Z.
$$\overline{A B}$$ || $$\overline{D E}$$, $$\overline{B C}$$ || $$\overline{E F}$$ Given
mâˆ ABC=mâˆ EZC If parallel lines are cut by a transversal, then corresponding angles are equal in measure.
mâˆ EZC=mâˆ DEF If parallel lines are cut by a transversal, then corresponding angles are equal in measure.
mâˆ ABC=mâˆ DEF Transitive property

Question 2.
In the figure to the right, $$\overline{A B}$$âˆ¥$$\overline{C D}$$. Prove that mâˆ AEC = aÂ° + cÂ°.

Draw a line through E parallel to $$\overline{A B}$$ and $$\overline{C D}$$.
$$\overline{A B}$$âˆ¥$$\overline{C D}$$ Given
mâˆ BAE=mâˆ AEF If parallel lines are cut by a transversal, then alternate interior angles are equal in measure.
mâˆ DCE=mâˆ FEC If parallel lines are cut by a transversal, then alternate interior angles are congruent equal in measure.

### Eureka Math Geometry Module 1 Lesson 10 Exit Ticket Answer Key

Write a proof for each question.

Question 1.
In the figure to the right, $$\overline{A B}$$ || $$\overline{C D}$$. Prove that aÂ°=bÂ°.

Write in angles c and d.
$$\overline{A B}$$ || $$\overline{C D}$$ Given
aÂ°=cÂ° Vertical angles are equal in measure.
cÂ°=dÂ° If parallel lines are cut by a transversal, then alternate interior angles are equal in measure.
dÂ°=bÂ° Vertical angles are equal in measure.
aÂ°=bÂ° Substitution property of equality

Question 2.
Prove mâˆ p=mâˆ r.