1. GEOMETRY 4-6 Triangle Inequalities Warm Up Warm Up Lesson Presentation Lesson Presentation Lesson Quiz Lesson Quiz

2. GEOMETRY 4-6 Triangle Inequalities 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. GEOMETRY 4-6 Triangle Inequalities Apply inequalities in one triangle. Objectives

4. GEOMETRY 4-6 Triangle Inequalities The positions of the longest and shortest sides of a triangle are related to the positions of the largest and smallest angles.

5. GEOMETRY 4-6 Triangle Inequalities Example 1: Ordering Triangle Angle Measures Write the angles in order from smallest to largest. The angles from smallest to largest are F, H and G. The shortest side is, so the smallest angle is F. The longest side is, so the largest angle is G.

6. GEOMETRY 4-6 Triangle Inequalities TEACH! Example 1 Write the angles in order from smallest to largest. The angles from smallest to largest are B, A, and C. The shortest side is, so the smallest angle is B. The longest side is, so the largest angle is C.

7. GEOMETRY 4-6 Triangle Inequalities Example 2: Ordering Triangle Side Lengths 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

8. GEOMETRY 4-6 Triangle Inequalities TEACH! Example 2 Write the sides in order from shortest to longest. mE = 180° – (90° + 22°) = 68° The smallest angle is D, so the shortest side is. The largest angle is F, so the longest side is. The sides from shortest to longest are

9. GEOMETRY 4-6 Triangle Inequalities A triangle is formed by three segments, but not every set of three segments can form a triangle.

10. GEOMETRY 4-6 Triangle Inequalities A certain relationship must exist among the lengths of three segments in order for them to form a triangle.

11. GEOMETRY 4-6 Triangle Inequalities Example 3: 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.

12. GEOMETRY 4-6 Triangle Inequalities TEACH! Example 3 Tell whether a triangle can have sides with the given lengths. Explain. 8, 13, 21 No—by the Triangle Inequality Theorem, a triangle cannot have these side lengths.

13. GEOMETRY 4-6 Triangle Inequalities Example 4: 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.

14. GEOMETRY 4-6 Triangle Inequalities TEACH! Example 4 Tell whether a triangle can have sides with the given lengths. Explain. 6.2, 7, 9 Yes—the sum of each pair of lengths is greater than the third side.

15. GEOMETRY 4-6 Triangle Inequalities Example 5: Applying the Triangle Inequality Theorem Tell whether a triangle can have sides with the given lengths. Explain. n + 6, n 2 – 1, 3n, when n = 4. Step 1 Evaluate each expression when n = 4. n + 6 4 + 6 10 n 2 – 1 (4) 2 – 1 15 3n3n 3(4) 12

16. GEOMETRY 4-6 Triangle Inequalities Example 5 Continued Step 2 Compare the lengths. Yes—the sum of each pair of lengths is greater than the third length.

17. GEOMETRY 4-6 Triangle Inequalities TEACH! Example 5 Tell whether a triangle can have sides with the given lengths. Explain. t – 2, 4t, t 2 + 1, when t = 4 Step 1 Evaluate each expression when t = 4. t – 2 4 – 2 2 t 2 + 1 (4) 2 + 1 17 4t4t 4(4) 16

18. GEOMETRY 4-6 Triangle Inequalities TEACH! Example 5 Continued Step 2 Compare the lengths. Yes—the sum of each pair of lengths is greater than the third length.

19. GEOMETRY 4-6 Triangle Inequalities 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. Combine the inequalities. So 5 < x < 21. The length of the third side is greater than 5 inches and less than 21 inches. x + 8 > 13 x > 5 x + 13 > 8 x > –5 8 + 13 > x 21 > x

20. GEOMETRY 4-6 Triangle Inequalities 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. Combine the inequalities. So 5 < x < 39. The length of the third side is greater than 5 inches and less than 39 inches. x + 22 > 17 x > –5 x + 17 > 22 x > 5 22 + 17 > x 39 > x

21. GEOMETRY 4-6 Triangle Inequalities 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 > 5 x + 51 > 46 x > –5 46 + 51 > x 97 > x 5 < x < 97 Combine the inequalities. Δ Inequal. Thm. Subtr. Prop. of Inequal. The distance from San Francisco to Oakland is greater than 5 miles and less than 97 miles.

22. GEOMETRY 4-6 Triangle Inequalities Check It Out! Example 5 The distance from San Marcos to Johnson City is 50 miles, and the distance from Seguin to San Marcos is 22 miles. What is the range of distances from Seguin to Johnson City? Let x be the distance from Seguin to Johnson City. x + 22 > 50 x > 28 x + 50 > 22 x > –28 22 + 50 > x 72 > x 28 < x < 72 Combine the inequalities. Δ Inequal. Thm. Subtr. Prop. of Inequal. The distance from Seguin to Johnson City is greater than 28 miles and less than 72 miles.

23. GEOMETRY 4-6 Triangle Inequalities Lesson-4 Quiz: Part I Use the diagram for Items 1–3. Find each measure. 1. ED 2. AB 3. mBFE 10 14 44°

24. GEOMETRY 4-6 Triangle Inequalities Lesson-4 Quiz: Part II 4. Find the value of n. 5. ∆XYZ is the midsegment triangle of ∆WUV. What is the perimeter of ∆XYZ? 16 11.5

25. GEOMETRY 4-6 Triangle Inequalities 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

26. GEOMETRY 4-6 Triangle Inequalities 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. No; 2.7 + 3.5 is not greater than 9.8. 5 cm < x < 29 cm 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.