5-5 Indirect Proof and Inequalities in One Triangle · Indirect Proof and Inequalities in One...

Holt McDougal Geometry

5-5 Indirect Proof and Inequalities

in One Triangle 5-5 Indirect Proof and Inequalities

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Holt Geometry

Warm Up

Lesson Presentation

Lesson Quiz




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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



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Write indirect proofs.

Apply inequalities in one triangle.

Objectives



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indirect proof

Vocabulary



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So far you have written proofs using direct reasoning. You began with a true hypothesis and built a logical argument to show that a conclusion was true. In an indirect proof, you begin by assuming that the conclusion is false. Then you show that this assumption leads to a contradiction. This type of proof is also called a proof by contradiction.



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When writing an indirect proof, look for a contradiction of one of the following: the given information, a definition, a postulate, or a theorem.

Helpful Hint



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Example 1: Writing an Indirect Proof

Step 1 Identify the conjecture to be proven.

Given: a > 0

Step 2 Assume the opposite of the conclusion.

Write an indirect proof that if a > 0, then

Prove:

Assume



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Example 1 Continued

Step 3 Use direct reasoning to lead to a contradiction.

However, 1 > 0.

1 0

Given, opposite of conclusion

Zero Prop. of Mult. Prop. of Inequality

Simplify.



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Step 4 Conclude that the original conjecture is true.

Example 1 Continued

The assumption that is false.

Therefore



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Check It Out! Example 1

Write an indirect proof that a triangle cannot have two right angles.

Step 1 Identify the conjecture to be proven.

Given: A triangle’s interior angles add up to 180°.

Prove: A triangle cannot have two right angles.

Step 2 Assume the opposite of the conclusion.

An angle has two right angles.



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Check It Out! Example 1 Continued

Step 3 Use direct reasoning to lead to a contradiction.

However, by the Protractor Postulate, a triangle cannot have an angle with a measure of 0°.

m1 + m2 + m3 = 180°

90° + 90° + m3 = 180°

180° + m3 = 180°

m3 = 0°



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Step 4 Conclude that the original conjecture is true.

The assumption that a triangle can have two right angles is false.

Therefore a triangle cannot have two right angles.

Check It Out! Example 1 Continued



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



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Example 2A: Ordering Triangle Side Lengths and

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.



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Example 2B: Ordering Triangle Side Lengths and

Angle Measures

Write the sides in order from shortest to longest.

mR = 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



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Check It Out! Example 2a

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.



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Check It Out! Example 2b

Write the sides in order from shortest to longest.

mE = 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



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



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



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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.



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Example 3B: Applying the Triangle Inequality

Theorem


2.3, 3.1, 4.6

Yes—the sum of each pair of lengths is greater than the third length.



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Example 3C: Applying the Triangle Inequality

Theorem


n + 6, n2 – 1, 3n, when n = 4.

Step 1 Evaluate each expression when n = 4.

n + 6

4 + 6

10

n2 – 1

(4)2 – 1

15

3n

3(4)

12



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Example 3C Continued

Step 2 Compare the lengths.




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Check It Out! Example 3a


8, 13, 21

No—by the Triangle Inequality Theorem, a triangle cannot have these side lengths.



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Check It Out! Example 3b


6.2, 7, 9

Yes—the sum of each pair of lengths is greater than the third side.



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Check It Out! Example 3c


t – 2, 4t, t2 + 1, when t = 4

Step 1 Evaluate each expression when t = 4.

t – 2

4 – 2

2

t2 + 1

(4)2 + 1

17

4t

4(4)

16



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Check It Out! Example 3c Continued

Step 2 Compare the lengths.




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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



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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



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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.



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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.



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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



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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.

5-5 Indirect Proof and Inequalities in One Triangle · Indirect Proof and Inequalities in One...

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