1. Five-Minute Check (over Lesson 5–4) Mathematical Practices Then/Now
Theorem 5.11: Triangle Inequality Theorem
Example 1: Identify Possible Triangles Given Side Lengths
Example 2: Standardized Test Example: Find Possible Side Lengths
Example 3: Real-World Example: Proof Using Triangle Inequality Theorem
Lesson Menu

2. A.  ABC is a right triangle. B.  A =  D
State the assumption you would make to start an indirect proof of the statement. ΔABC  ΔDEF
A.  ABC is a right triangle.
B.  A =  D
C. AB = DE
D. ΔABC / ΔDEF
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5-Minute Check 1

3. A. RS is a perpendicular bisector. B. RS is not an angle bisector.
State the assumption you would make to start an indirect proof of the statement. RS is an angle bisector.
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A. RS is a perpendicular bisector.
B. RS is not an angle bisector.
C. R is the midpoint of ST.
D. m R = 90°
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5-Minute Check 2

4. A.  X is a not a right angle. B. m X < 90°
State the assumption you would make to start an indirect proof of the statement.  X is a right angle.
A.  X is a not a right angle.
B. m X < 90°
C. m X > 90°
D. m X = 90°
5-Minute Check 3

5. State the assumption you would make to start an indirect proof of the statement. If 4x – 3 ≤ 9, then x ≤ 3.
A. 4x – 3 ≤ 9
B. x > 3
C. x > 1
D. 4x ≤ 6
5-Minute Check 4

6. A. ΔMNO is a right triangle. B. ΔMNO is an isosceles triangle.
State the assumption you would make to start an indirect proof of the statement. ΔMNO is an equilateral triangle.
A. ΔMNO is a right triangle.
B. ΔMNO is an isosceles triangle.
C. ΔMNO is not an equilateral triangle.
D. MN = NO = MO
5-Minute Check 5

7. Which statement is a contradiction to the statement that AB  CD?
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A. AB = CD
B. AB > CD
C. CD  AB
D. AB ≤ CD
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5-Minute Check 6

8. Mathematical Practices
1 Make sense of problems and persevere in solving them.
2 Reason abstractly and quantitatively.
Content Standards
G.CO.10 Prove theorems about triangles.
G.MG.3 Apply geometric methods to solve problems (e.g., designing an object or structure to satisfy physical constraints or minimize cost; working with graphical grid systems based on ratios). MP

9. Use the Triangle Inequality Theorem to identify possible triangles.
You recognized and applied properties of inequalities to the relationships between the angles and sides of a triangle.
Use the Triangle Inequality Theorem to identify possible triangles.
Prove triangle relationships using the Triangle Inequality Theorem.
Then/Now

10. Concept

11. Identify Possible Triangles Given Side Lengths
A. Is it possible to form a triangle with side lengths of 6 , , and ? If not, explain why not. __ 1 2
Check each inequality.  X
Answer:
Example 1

12. Identify Possible Triangles Given Side Lengths
B. Is it possible to form a triangle with side lengths of 6.8, 7.2, 5.1? If not, explain why not.
Check each inequality.
> > > 7.2
14 > 5.1 12.3 > 6.8  > 7.2 
Since the sum of all pairs of side lengths are greater than the third side length, sides with lengths 6.8, 7.2, and 5.1 will form a triangle.
Answer: yes
Example 1

13. A. yes
B. no
Example 1

14. B. Is it possible to form a triangle given the side lengths 4. 8, 12
B. Is it possible to form a triangle given the side lengths 4.8, 12.2, and 15.1?
A. yes
B. no
Example 1

15. In ΔPQR, PQ = 7.2 and QR = 5.2. Which measure cannot be PR? A 7 B 9
Find Possible Side Lengths
In ΔPQR, PQ = 7.2 and QR = 5.2. Which measure cannot be PR?
A 7
B 9
C 11
D 13
Example 2

16. You need to determine which value is not valid.
Find Possible Side Lengths
Read the Test Item
You need to determine which value is not valid.
Solve the Test Item
Solve each inequality to determine the range of values for PR. or
n < 12.4
Example 2

17. Find Possible Side Lengths
Notice that n > –2 is always true for any whole number measure for x. Combining the two remaining inequalities, the range of values that fit both inequalities is n > 2 and n < 12.4, which can be written as 2 < n < 12.4.
Example 2

18. Find Possible Side Lengths
Examine the answer choices. The only value that does not satisfy the compound inequality is 13 since 13 is greater than Thus, the answer is choice D.
Answer: D
Example 2

19. In ΔXYZ, XY = 6, and YZ = 9. Which measure cannot be XZ?
Example 2

20. Proof Using Triangle Inequality Theorem
TRAVEL The towns of Jefferson, Kingston, and Newbury are shown in the map below. Prove that driving first from Jefferson to Kingston and then Kingston to Newbury is a greater distance than driving from Jefferson to Newbury.
Example 3

21. Proof Using Triangle Inequality Theorem
Abbreviating the vertices as J, K, and N: JK represents the distance from Jefferson to Kingstown; KN represents the distance from Kingston to Newbury; and JN the distance from Jefferson to Newbury.
Answer: By the Triangle Inequality Theorem, JK + KN > JN. Therefore, driving from Jefferson to Kingston and then Kingston to Newbury is a greater distance than driving from Jefferson to Newbury.
Example 3

22. A. Jacinda is correct, HC + CG > HG.
Jacinda is trying to run errands around town. She thinks it is a longer trip to drive to the cleaners and then to the grocery store, than to the grocery store alone. Determine whether Jacinda is right or wrong.
A. Jacinda is correct, HC + CG > HG.
B. Jacinda is not correct, HC + CG < HG.
Example 3