Indirect Proof
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95698_ch02_067-120.qxp 9/5/13 4:53 PM Page 76 76 CHAPTER 2 ■ PARALLEL LINES In Exercises 28 to 30, write a formal proof of each theorem. 34. Given: Triangle MNQ with M obtuse ∠MNQ 28. If two parallel lines are cut by a transversal, then the pairs Construct: NE Ќ MQ, with E of alternate exterior angles are congruent. on MQ. N Q 29. If two parallel lines are cut by a transversal, then the pairs 35. Given: Triangle MNQ with of exterior angles on the same side of the transversal are Exercises 34, 35 obtuse ∠MNQ supplementary. Construct: MR Ќ NQ 30. If a transversal is perpendicular to one of two parallel lines, (HINT: Extend NQ.) then it is also perpendicular to the other line. 36. Given: A line m and a point T not on m 31. Suppose that two lines are cut by a transversal in such a way that the pairs of corresponding angles are not congruent. T Can those two lines be parallel? 32. Given: Line ᐍ and point P P m not on ᐍ —! Suppose that you do the following: Construct: PQ Ќ ᐍ i) Construct a perpendicular line r from T to line m. ii) Construct a line s perpendicular to line r at point T. 33. Given: Triangle ABC with B three acute angles What is the relationship between lines s and m? Construct: BD Ќ AC, with D on AC. A C 2.2 Indirect Proof KEY CONCEPTS Conditional Contrapositive Indirect Proof Converse Law of Negative Inverse Inference Let P S Q represent the conditional statement “If P, then Q.” The following statements are related to this conditional statement (also called an implication). NOTE: Recall that ~P represents the negation of P. Conditional (or Implication) P S Q If P, then Q. Converse of Conditional Q S P If Q, then P. Inverse of Conditional ~P S ~Q If not P, then not Q. Contrapositive of Conditional ~Q S ~P If not Q, then not P. Consider the following conditional statement. If Tom lives in San Diego, then he lives in California. This true conditional statement has the following related statements: Converse: If Tom lives in California, then he lives in San Diego. (false) Inverse: If Tom does not live in San Diego, then he does not live in California. (false) Contrapositive: If Tom does not live in California, then he does not live in San Diego. (true) Unless otherwise noted, all content on this page is © Cengage Learning. Copyright 2014 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 95698_ch02_067-120.qxp 9/5/13 4:54 PM Page 77 2.2 ■ Indirect Proof 77 In general, the conditional statement and its contrapositive are either both true or both false! Similarly, the converse and the inverse are also either both true or both false. EXAMPLE 1 For the conditional statement that follows, give the converse, the inverse, and the con- trapositive. Then classify each as true or false. If two angles are vertical angles, then they are congruent angles. SOLUTION CONVERSE: If two angles are congruent angles, then they are vertical angles. (false) INVERSE: If two angles are not vertical angles, then they are not congruent angles. (false) CONTRAPOSITIVE: If two angles are not congruent angles, then they are not vertical angles. (true) “If P, then Q” and “If not Q, then not P” are equivalent. P Venn Diagrams can be used to explain why the conditional statement P S Q and its Q contrapositive ~Q S ~P are equivalent. The relationship “If P, then Q” is represented in Figure 2.11. Note that if any point is selected outside of Q (that is ~Q), then it cannot Figure 2.11 possibly lie in set P (thus, ϳ P). THE LAW OF NEGATIVE INFERENCE (CONTRAPOSITION) EXS. 1, 2 Consider the following circumstances, and accept each premise as true: 1. If Matt cleans his room, then he will go to the movie. (P S Q ) 2. Matt does not get to go to the movie. (~Q ) What can you conclude? You should have deduced that Matt did not clean his room; if he had, he would have gone to the movie. This “backdoor” reasoning is based on the fact that the truth of P S Q implies the truth of ~Q S ~P. LAW OF NEGATIVE INFERENCE (CONTRAPOSITION) 1. P S Q 2. ~ Q C. І ϳ P EXAMPLE 2 Use the Law of Negative Inference to draw a valid conclusion for this argument. 1. If the weather is nice Friday, we will go on a picnic. 2. We did not go on a picnic Friday. C. І ? SOLUTION The weather was not nice Friday. Unless otherwise noted, all content on this page is © Cengage Learning. Copyright 2014 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 95698_ch02_067-120.qxp 9/12/13 11:29 AM Page 78 78 CHAPTER 2 ■ PARALLEL LINES Like the Law of Detachment from Section 1.1, the Law of Negative Inference (Law of Contraposition) is a form of deduction. Whereas the Law of Detachment characterizes the method of “direct proof” found in preceding sections, the Law of Negative Inference char- acterizes the method of proof known as indirect proof. INDIRECT PROOF EXS. 3, 4 You will need to know when to use the indirect method of proof. Often the theorem to be proved has the form P S Q, in which Q is a negation and denies some claim. For instance, an indirect proof might be best if Q reads in one of these ways: c is not equal to d ᐍ is not perpendicular to m However, we will see in Example 5 of this section that the indirect method can be used to prove that line ᐍ is parallel to line m. Indirect proof is also used for proving existence and uniqueness theorems; see Example 6. Geometry in the Real World The method of indirect proof is illustrated in Example 3. All indirect proofs in this book are given in paragraph form (as are some of the direct proofs). In any paragraph proof, each statement must still be justified. Because of the need to order your statements properly, writing any type of proof may have a positive impact on the essays you write for your other classes! EXAMPLE 3 ! ! GIVEN: In Figure 2.12, BA is not perpendicular to BD When the bubble displayed on the level is not centered, the board PROVE:∠1 and ∠2 are not complementary A used in construction is neither PROOF: Suppose that ∠1 and ∠2 are vertical nor horizontal. complementary. Then m∠1 ϩ m∠2 ϭ 90Њ C because the sum of the measures of two complementary ∠s is 90. We also know that 1 m∠1 ϩ m∠2 ϭ m∠ABD by the Angle- 2 B Addition Postulate. In turn, m∠ABD ϭ 90Њ D by substitution.! ! Then ∠ABD is a right angle. In Figure 2.12 turn, BA Ќ BD. But this contradicts the given hypothesis; therefore, the supposition must be false, and it follows that ∠1 and ∠2 are not complementary. In Example 3 and in all indirect proofs, the first statement takes the form Suppose/Assume the exact opposite of the Prove Statement. By its very nature, such a statement cannot be supported even though every other statement in the proof can be justified; thus, when a contradiction is reached, the finger of blame points to the supposition. Having reached a contradiction, we may say that the claim involving ~Q has failed and is false; in effect, the double negative ~(~Q) is equivalent to Q. Thus, our only recourse is to conclude that Q is true. Following is an outline of this technique. Unless otherwise noted, all content on this page is © Cengage Learning. Copyright 2014 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 95698_ch02_067-120.qxp 9/5/13 4:54 PM Page 79 2.2 ■ Indirect Proof 79 STRATEGY FOR PROOF ■ Method of Indirect Proof EXS. 5–7 To prove the statement P S Q or to complete the proof problem of the form Given: P Prove: Q by the indirect method, use the following steps: 1. Suppose that ~Q is true. 2. Reason from the supposition until you reach a contradiction. 3. Note that the supposition claiming that ~Q is true must be false and that Q must therefore be true. Step 3 completes the proof. The contradiction found in an indirect proof often takes the form “Q and ~Q,” which cannot be true.