1. 5-5 Indirect Proof and Inequalities in One Triangle Warm Up
Lesson Presentation
Lesson Quiz
Holt Geometry

2. Warm Up
1. Write a conditional from the sentence “An isosceles triangle has two congruent sides.”
2. Write the contrapositive of the conditional “If it is Tuesday, then John has a piano lesson.”
3. Show that the conjecture “If x > 6, then 2x > 14” is false by finding a counterexample.
If a ∆ is isosc., then it has 2  sides.
If John does not have a piano lesson, then it is not Tuesday.
x = 7

3. Learning Targets
Apply inequalities in one triangle.

4. The positions of the longest and shortest sides of a triangle are related to the positions of the largest and smallest angles.

5. Example 2A: Ordering Triangle Side Lengths and Angle Measures
Write the angles in order from smallest to largest.
The shortest side is , so the smallest angle is F.
The longest side is , so the largest angle is G.
The angles from smallest to largest are F, H and G.

6. Example 2B: Ordering Triangle Side Lengths and Angle Measures
Write the sides in order from shortest to longest.
mR = 180° – (60° + 72°) = 48°
The smallest angle is R, so the shortest side is .
The largest angle is Q, so the longest side is .
The sides from shortest to longest are

7. A triangle is formed by three segments, but not every set of three segments can form a triangle.

8. A certain relationship must exist among the lengths of three segments in order for them to form a triangle.

9. Example 3A: Applying the Triangle Inequality Theorem
Tell whether a triangle can have sides with the given lengths. Explain.
7, 10, 19
No—by the Triangle Inequality Theorem, a triangle cannot have these side lengths.

10. Example 3B: Applying the Triangle Inequality Theorem
Tell whether a triangle can have sides with the given lengths. Explain.
2.3, 3.1, 4.6   
Yes—the sum of each pair of lengths is greater than the third length.

11. Example 3C: Applying the Triangle Inequality Theorem
Tell whether a triangle can have sides with the given lengths. Explain.
n + 6, n2 – 1, 3n, when n = 4.
Step 1 Evaluate each expression when n = 4.
n + 6
n2 – 1 3n
4 + 6
(4)2 – 1
3(4) 10 15 12

12. Example 3C Continued
Step 2 Compare the lengths.   
Yes—the sum of each pair of lengths is greater than the third length.

13. Example 4: Finding Side Lengths
The lengths of two sides of a triangle are 8 inches and 13 inches. Find the range of possible lengths for the third side.
Let x represent the length of the third side. Then apply the Triangle Inequality Theorem.
x + 8 > 13
x + 13 > 8
> x
x > 5
x > –5
21 > x
Combine the inequalities. So 5 < x < 21. The length of the third side is greater than 5 inches and less than 21 inches.

14. Check It Out! Example 4
The lengths of two sides of a triangle are 22 inches and 17 inches. Find the range of possible lengths for the third side.
Let x represent the length of the third side. Then apply the Triangle Inequality Theorem.
x + 22 > 17
x + 17 > 22
> x
x > –5
x > 5
39 > x
Combine the inequalities. So 5 < x < 39. The length of the third side is greater than 5 inches and less than 39 inches.

15. Example 5: Travel Application
The figure shows the approximate distances between cities in California. What is the range of distances from San Francisco to Oakland?
Let x be the distance from San Francisco to Oakland.
x + 46 > 51
x + 51 > 46
> x
Δ Inequal. Thm.
x > 5
x > –5
97 > x
Subtr. Prop. of Inequal.
5 < x < 97
Combine the inequalities.
The distance from San Francisco to Oakland is greater than 5 miles and less than 97 miles.

16. Lesson Quiz: Part I
1. Write the angles in order from smallest to largest.
2. Write the sides in order from shortest to longest.
C, B, A

17. Lesson Quiz: Part II
3. The lengths of two sides of a triangle are 17 cm and 12 cm. Find the range of possible lengths for the third side.
4. Tell whether a triangle can have sides with lengths 2.7, 3.5, and 9.8. Explain.
5 cm < x < 29 cm
No; is not greater than 9.8.
5. Ray wants to place a chair so it is 10 ft from his television set. Can the other two distances
shown be 8 ft and 6 ft? Explain.
Yes; the sum of any two lengths is greater than the third length.