1. Inequalities in Two Triangles
5-6
Inequalities in Two Triangles
Warm Up
Lesson Presentation
Lesson Quiz
Holt Geometry

2. Warm Up 5.6 Indirect Proof and Inequalities in Two Triangles
1. Write the angles in order from smallest to largest.
2. The lengths of two sides of a triangle are 12 cm and 9 cm. Find the range of possible lengths for the third side.
X, Z, Y
3 cm < s < 21 cm

3. Objective Apply inequalities in two triangles.
5.6 Indirect Proof and Inequalities in Two Triangles
Objective
Apply inequalities in two triangles.

4. 5.6 Indirect Proof and Inequalities in Two Triangles

5. Example 1A: Using the Hinge Theorem and Its Converse
5.6 Indirect Proof and Inequalities in Two Triangles
Example 1A: Using the Hinge Theorem and Its Converse
Compare mBAC and mDAC.
Compare the side lengths in ∆ABC and ∆ADC.
AB = AD AC = AC BC > DC
By the Converse of the Hinge Theorem, mBAC > mDAC.

6. Example 1B: Using the Hinge Theorem and Its Converse
5.6 Indirect Proof and Inequalities in Two Triangles
Example 1B: Using the Hinge Theorem and Its Converse
Compare EF and FG.
Compare the sides and angles in ∆EFH angles in ∆GFH.
mGHF = 180° – 82° = 98°
EH = GH FH = FH mEHF > mGHF
By the Hinge Theorem, EF < GF.

7. Example 1C: Using the Hinge Theorem and Its Converse
5.6 Indirect Proof and Inequalities in Two Triangles
Example 1C: Using the Hinge Theorem and Its Converse
Find the range of values for k.
Step 1 Compare the side lengths in ∆MLN and ∆PLN.
LN = LN LM = LP MN > PN
By the Converse of the Hinge Theorem, mMLN > mPLN.
5k – 12 < 38
Substitute the given values.
k < 10
Add 12 to both sides and divide by 5.

8. 5.6 Indirect Proof and Inequalities in Two Triangles
Example 1C Continued
Step 2 Since PLN is in a triangle, mPLN > 0°.
5k – 12 > 0
Substitute the given values.
k < 2.4
Add 12 to both sides and divide by 5.
Step 3 Combine the two inequalities.
The range of values for k is 2.4 < k < 10.

9. 5.6 Indirect Proof and Inequalities in Two Triangles
Check It Out! Example 1a
Compare mEGH and mEGF.
Compare the side lengths in ∆EGH and ∆EGF.
FG = HG EG = EG EF > EH
By the Converse of the Hinge Theorem, mEGH < mEGF.

10. 5.6 Indirect Proof and Inequalities in Two Triangles
Check It Out! Example 1b
Compare BC and AB.
Compare the side lengths in ∆ABD and ∆CBD.
AD = DC BD = BD mADB > mBDC.
By the Hinge Theorem, BC > AB.

11. Example 2: Travel Application
5.6 Indirect Proof and Inequalities in Two Triangles
Example 2: Travel Application
John and Luke leave school at the same time. John rides his bike 3 blocks west and then 4 blocks north. Luke rides 4 blocks east and then 3 blocks at a bearing of N 10º E. Who is farther from school? Explain.

12. 5.6 Indirect Proof and Inequalities in Two Triangles
Example 2 Continued
The distances of 3 blocks and 4 blocks are the same in both triangles.
The angle formed by John’s route (90º) is smaller than the angle formed by Luke’s route (100º). So Luke is farther from school than John by the Hinge Theorem.

13. 5.6 Indirect Proof and Inequalities in Two Triangles
Check It Out! Example 2
When the swing ride is at full speed, the chairs are farthest from the base of the swing tower. What can you conclude about the angles of the swings at full speed versus low speed? Explain.
The  of the swing at full speed is greater than the  at low speed because the length of the triangle on the opposite side is the greatest at full swing.

14. Example 3: Proving Triangle Relationships
5.6 Indirect Proof and Inequalities in Two Triangles
Example 3: Proving Triangle Relationships
Write a two-column proof.
Given:
Prove: AB > CB
Proof:
Statements
Reasons
1. Given
2. Reflex. Prop. of 
3. Hinge Thm.

15. 5.6 Indirect Proof and Inequalities in Two Triangles
Check It Out! Example 3a
Write a two-column proof.
Given: C is the midpoint of BD.
m1 = m2
m3 > m4
Prove: AB > ED

16. 5.6 Indirect Proof and Inequalities in Two Triangles
Statements
Reasons
1. C is the mdpt. of BD
m3 > m4,
m1 = m2
1. Given
2. Def. of Midpoint
3. 1  2
3. Def. of  s
4. Conv. of Isoc. ∆ Thm.
5. AB > ED
5. Hinge Thm.

17. 5.6 Indirect Proof and Inequalities in Two Triangles
Check It Out! Example 3b
Write a two-column proof.
Given:
SRT  STR
TU > RU
Prove: mTSU > mRSU
Statements
Reasons
1. SRT  STR
TU > RU
1. Given
2. Conv. of Isoc. Δ Thm.
3. Reflex. Prop. of 
4. mTSU > mRSU
4. Conv. of Hinge Thm.

18. 5.6 Indirect Proof and Inequalities in Two Triangles
Lesson Quiz: Part I
1. Compare mABC and mDEF.
2. Compare PS and QR.
mABC > mDEF
PS < QR

19. 5.6 Indirect Proof and Inequalities in Two Triangles
Lesson Quiz: Part II
3. Find the range of values for z.
–3 < z < 7

20. 5.6 Indirect Proof and Inequalities in Two Triangles
Lesson Quiz: Part III
4. Write a two-column proof.
Prove: mXYW < mZWY
Given:
Proof:
Statements
Reasons
1. Given
2. Reflex. Prop. of 
3. Conv. of Hinge Thm.
3. mXYW < mZWY