2. Example 1A: Using the Converse of the Corresponding Angles Postulate
Use the Converse of the Corresponding Angles Postulate and the given information to show that ℓ || m.
4  8
4  8 4 and 8 are corresponding angles.
ℓ || m Conv. of Corr. s Post.

3. Example 1B: Using the Converse of the Corresponding Angles Postulate
Use the Converse of the Corresponding Angles Postulate and the given information to show that ℓ || m.
m3 = (4x – 80)°,
m7 = (3x – 50)°, x = 30
m3 = 4(30) – 80 = 40 Substitute 30 for x.
m8 = 3(30) – 50 = 40 Substitute 30 for x.
m3 = m8 Trans. Prop. of Equality
3   Def. of  s.
ℓ || m Conv. of Corr. s Post.

4. Check It Out! Example 1a
Use the Converse of the Corresponding Angles Postulate and the given information to show that ℓ || m.
m1 = m3
1  3 1 and 3 are corresponding angles.
ℓ || m Conv. of Corr. s Post.

5. Check It Out! Example 1b
Use the Converse of the Corresponding Angles Postulate and the given information to show that ℓ || m.
m7 = (4x + 25)°,
m5 = (5x + 12)°, x = 13
m7 = 4(13) + 25 = 77 Substitute 13 for x.
m5 = 5(13) + 12 = 77 Substitute 13 for x.
m7 = m5 Trans. Prop. of Equality
7   Def. of  s.
ℓ || m Conv. of Corr. s Post.

6. The Converse of the Corresponding Angles Postulate is used to construct parallel lines. The Parallel Postulate guarantees that for any line ℓ, you can always construct a parallel line through a point that is not on ℓ.

8. Example 2A: Determining Whether Lines are Parallel
Use the given information and the theorems you have learned to show that r || s.
4  8
4  8 4 and 8 are alternate exterior angles.
r || s Conv. Of Alt. Int. s Thm.

9. Example 2B: Determining Whether Lines are Parallel
Use the given information and the theorems you have learned to show that r || s.
m2 = (10x + 8)°,
m3 = (25x – 3)°, x = 5
m2 = 10x + 8
= 10(5) + 8 = 58 Substitute 5 for x.
m3 = 25x – 3
= 25(5) – 3 = 122 Substitute 5 for x.

10. Example 2B Continued
Use the given information and the theorems you have learned to show that r || s.
m2 = (10x + 8)°,
m3 = (25x – 3)°, x = 5
m2 + m3 = 58° + 122°
= 180° 2 and 3 are same-side interior angles.
r || s Conv. of Same-Side Int. s Thm.

11. Check It Out! Example 2a
Refer to the diagram. Use the given information and the theorems you have learned to show that r || s.
m4 = m8
4  8 Congruent angles
4  8 4 and 8 are alternate exterior angles.
r || s Conv. of Alt. Int. s Thm.

12. Check It Out! Example 2b
Refer to the diagram. Use the given information and the theorems you have learned to show that r || s.
m3 = 2x, m7 = (x + 50), x = 50
m3 = 2x
= 2(50) = 100° Substitute 50 for x.
m7 = x + 50
= = 100° Substitute 5 for x.
m3 = 100 and m7 = 100
3  7
r||s Conv. of the Alt. Int. s Thm.

13. Example 3: Proving Lines Parallel
Given: p || r , 1  3
Prove: ℓ || m

14. Example 3 Continued
Statements
Reasons
1. p || r
1. Given
2. 3  2
2. Alt. Ext. s Thm.
3. 1  3
3. Given
4. 1  2
4. Trans. Prop. of 
5. ℓ ||m
5. Conv. of Corr. s Post.

15. Check It Out! Example 3
Given: 1  4, 3 and 4 are supplementary.
Prove: ℓ || m

16. Check It Out! Example 3 Continued
Statements
Reasons
1. 1  4
1. Given
2. m1 = m4
2. Def.  s
3. 3 and 4 are supp.
3. Given
4. m3 + m4 = 180
4. Trans. Prop. of 
5. m3 + m1 = 180
5. Substitution
6. m2 = m3
6. Vert.s Thm.
7. m2 + m1 = 180
7. Substitution
8. ℓ || m
8. Conv. of Same-Side Interior s Post.

17. Example 4: Carpentry Application
A carpenter is creating a woodwork pattern and wants two long pieces to be parallel. m1= (8x + 20)° and m2 = (2x + 10)°. If x = 15, show that pieces A and B are parallel.

18. Substitute 15 for x in each expression.
Example 4 Continued
A line through the center of the horizontal piece forms a transversal to pieces A and B.
1 and 2 are same-side interior angles. If 1 and 2 are supplementary, then pieces A and B are parallel.
Substitute 15 for x in each expression.

19. Example 4 Continued
m1 = 8x + 20
= 8(15) + 20 = 140
Substitute 15 for x.
m2 = 2x + 10
= 2(15) + 10 = 40
Substitute 15 for x.
m1+m2 =
1 and 2 are supplementary.
= 180
The same-side interior angles are supplementary, so pieces A and B are parallel by the Converse of the Same-Side Interior Angles Theorem.

20. Check It Out! Example 4
What if…? Suppose the corresponding angles on the opposite side of the boat measure (4y – 2)° and (3y + 6)°, where
y = 8. Show that the oars are parallel.
4y – 2 = 4(8) – 2 = 30° y + 6 = 3(8) + 6 = 30°
The angles are congruent, so the oars are || by the Conv. of the Corr. s Post.

21. Example 1: Distance From a Point to a Line
A. Name the shortest segment from point A to BC.
The shortest distance from a point to a line is the length of the perpendicular segment, so AP is the shortest segment from A to BC.
B. Write and solve an inequality for x.
AC > AP
AP is the shortest segment.
x – 8 > 12
Substitute x – 8 for AC and 12 for AP.
+ 8
Add 8 to both sides of the inequality.
x > 20

22. Check It Out! Example 1
A. Name the shortest segment from point A to BC.
The shortest distance from a point to a line is the length of the perpendicular segment, so AB is the shortest segment from A to BC.
B. Write and solve an inequality for x.
AC > AB
AB is the shortest segment.
12 > x – 5
Substitute 12 for AC and x – 5 for AB.
+ 5
Add 5 to both sides of the inequality.
17 > x

23. HYPOTHESIS
CONCLUSION

24. Example 2: Proving Properties of Lines
Write a two-column proof.
Given: r || s, 1  2
Prove: r  t

25. Example 2 Continued
Statements
Reasons
1. r || s, 1  2
1. Given
2. 2  3
2. Corr. s Post.
3. 1  3
3. Trans. Prop. of 
4. 2 intersecting lines form lin. pair of  s  lines .
4. r  t

26. Check It Out! Example 2
Write a two-column proof.
Given:
Prove:

27. Check It Out! Example 2 Continued
Statements
Reasons
1. EHF  HFG
1. Given 2.
2. Conv. of Alt. Int. s Thm. 3.
3. Given 4.
4.  Transv. Thm.

28. Example 3: Carpentry Application
A carpenter’s square forms a
right angle. A carpenter places
the square so that one side is
parallel to an edge of a board, and then draws a line along the other side of the square. Then he slides the square to the right and draws a second line. Why must the two lines be parallel?
Both lines are perpendicular to the edge of the board. If two coplanar lines are perpendicular to the same line, then the two lines are parallel to each other, so the lines must be parallel to each other.

29. Check It Out! Example 3
A swimmer who gets caught in a rip current should swim in a direction perpendicular to the current. Why should the path of the swimmer be parallel to the shoreline?

30. Check It Out! Example 3 Continued
The shoreline and the path of the swimmer should both be  to the current, so they should be || to each other.