When you were provided Postulate 10.1, you were able come prove numerous angle relationships that emerged when two parallel currently were reduced by a transversal. There are times when particular angle relationships are given to you, and you need to recognize whether or no the lines room parallel. You"ll develop some theorems to assist you perform this easily. Your very first theorem, theorem 10.7, will be developed using contradiction. The remainder of the theorems will follow utilizing a straight proof and Theorem 10.7.

You are watching: How to prove that two lines are parallel Figure 10.8l and m are reduced by a transversal t, ?1 and ?2 are matching angles.

Let"s testimonial the steps connected in building a proof by contradiction. Start by assuming that the conclusion is false, and then mirroring that the hypotheses must likewise be false. In the initial statement the the proof, you begin with congruent equivalent angles and conclude that the 2 lines room parallel. Come prove this theorem utilizing contradiction, assume that the 2 lines are not parallel, and also show that the matching angles cannot be congruent.

Figure 10.9l and m are cut by a transversal t, l ? ? m, r ? ? l, and also r, m, and l intersect at O.

Theorem 10.7: If two lines are reduced by a transversal so the the matching angles room congruent, then these lines are parallel.

A drawing of this case is presented in number 10.8. Two lines, l and m are cut by a transversal t, and also ?1 and ?2 are corresponding angles.

Given: l and also m are reduced by a transversal t, l ?/? m. Prove: ?1 and ?2 are not congruent (?1 ~/= ?2). Proof: Assume that l ?/? m. Due to the fact that l and m are reduced by a transversal t, m and also t have to intersect. Girlfriend might contact the allude of intersection the m and also t the point O. Because l is no parallel to m, we can find a line, say r, that passes v O and also is parallel come l. I"ve attracted this new line in number 10.9. In this brand-new drawing, ?3 and ?2 are equivalent angles, for this reason by Postulate 10.1, they room congruent. But wait a minute! If ?2 ~= ?3, and also m?3 + m?4 = m?1 by the Angle addition Postulate, m?2 + m?4 = m?1. Because m?4 > 0 (by the Protractor Postulate), this way that m?2 StatementsReasons
1. l and also m are two lines reduced by a transversal t, with 1 |/| mGiven
2. Let r be a heat passing v O which is parallel come lEuclid"s fifth postulate
3. ?3 and ?2 are equivalent anglesDefinition of matching angles
4. ?2 ~= ?3 Postulate 10.1
5. m?2 = m?3 an interpretation of ~=
6. m?3 + m?4 = m?1 Angle enhancement Postulate
7. m?2 + m?4 = m?1 Substitution (steps 5 and also 6)
8. m?4 > 0 Protractor Postulate
9. m?2 Definition the inequality
10. ?4 ~/= ?8 meaning of ~=

That completes her proof through contradiction. The rest of the theorems that you prove in this section will exploit Theorem 10.7. The rest of the theorems in this section are converses of theorems proved earlier.

Let"s take it a look in ~ some other angle relationships that can be used to prove that two lines room parallel. These 2 theorems space similar, and to be fair I will prove the very first one and leave you to prove the second.

Theorem 10.8: If two lines are cut by a transversal so that the alternating interior angles space congruent, climate these lines room parallel. Theorem 10.9: If two lines are cut by a transversal therefore that alternative exterior angles room congruent, climate these lines space parallel.

Figure 10.10 shows two lines reduced by a transversal t, with alternative interior angles labeled ?1 and also ?2.

Figure 10.10l and also m are cut by a transversal t, and ?1 and also ?2 are alternating interior angles.

Given: l and m are reduced by a transversal t, through ?4 ~= ?8. Prove: l ? ? m. Proof: The game setup is simple. In bespeak to use Theorem 10.7, you require to present that equivalent angles room congruent. You can use the fact that ?1 and ?2 space vertical angles, for this reason they space congruent. Due to the fact that ?2 and ?3 are corresponding angles, if you can show that they are congruent, then you will have the ability to conclude that your lines room parallel. The transitive property of congruence will placed the pond in the coffin, so to speak.StatementsReasons
1. l and m space two lines cut by a transversal t, through ?4 ~= ?8 Given
2. ?1 and also ?3 are vertical anglesDefinition of upright angles
3. ?1 ~= ?3 organize 8.1
4. ?2 and also ?3 are equivalent anglesDefinition of corresponding angles
5. ?2 ~= ?3 Transitive residential or commercial property of ~=
6. l ? ? mTheorem 10.7

Theorem 10.4 developed the truth that if 2 parallel currently are reduced by a transversal, then the internal angles on the exact same side of the transversal are supplementary angles. Theorem 10.5 claimed that if two parallel lines are reduced by a transversal, then the exterior angle on the same side the the transversal space supplementary angles. It"s now time come prove the converse of these statements. Let"s break-up the work: I"ll prove to organize 10.10 and also you"ll take care of to organize 10.11.

Theorem 10.10: If two lines are cut by a transversal so that the internal angles ~ above the same side that the transversal are supplementary, then these lines room parallel. Theorem 10.11: If two lines are cut by a transversal so the the exterior angle on the exact same side of the transversal room supplementary, then these lines space parallel.

Figure 10.11 will assist you visualize this situation. 2 lines, l and m, are reduced by a transversal t, with inner angles top top the very same side that the transversal labeling ?1 and also ?2.

Figure 10.11l and also m, are cut by a transversal t, and ?1 and also ?2 are interior angles on the very same side that the transversal.

Given: l and m are cut by a transversal t, ?1 and also ?2 are supplementary angles. Prove: l ? ? m. Proof: Here"s the game plan. In order to use Theorem 10.7, you require to present that matching angles room congruent. Yet it could be less complicated to usage Theorem 10.8 if you can show that ?2 and ?3 room congruent. You deserve to do that reasonably easily, if you apply what you discovered. Due to the fact that ?1 and also ?3 are supplementary angles, and ?1 and also ?2 are supplementary angles, you have the right to conclude the ?2 ~= ?3. Then you apply Theorem 10.8 and your job-related is done.StatementsReasons
1.l and m room two lines cut by a transversal t, ?1 and ?2 space supplementary angles.Given
2.?1 and ?3 room supplementary anglesDefinition that supplementary angles
3.?2 ~= ?3?1 and ?3 are supplementary angles, and ?1 and also ?2 are supplementary angles
4.l ? ? mTheorem 10.8

In a complex world, a complex theorem needs a facility drawing. If your drawing is too involved, it might be complicated to decision which lines are parallel due to the fact that of congruent angles. Think about Figure 10.12. Expect that ?1 ~= ?3. Which lines have to be parallel? because ?1 and also ?3 are corresponding angles when viewing currently o and also n cut by transversal m, o ? ? n.

Figure 10.12The intersection of currently l, m, n, and o.

Put Me in, Coach!

Here"s your possibility to shine. Mental that ns am v you in spirit and also have detailed the answers come these concerns in prize Key.

If together ? ? m together in figure 10.4, with m?2 = 2x - 45 and also m?1 = x, find m?6 and m?8. Write a formal proof because that Theorem 10.3. Write a officially proof because that Theorem 10.5. Prove to organize 10.9. Prove to organize 10.11. In figure 10.12, i beg your pardon lines need to be parallel if ?3 ~= ?11 ?

Excerpted indigenous The complete Idiot"s overview to Geometry 2004 through Denise Szecsei, Ph.D.. All rights reserved including the appropriate of reproduction in totality or in part in any kind of form. Provided by setup with Alpha Books, a member the Penguin group (USA) Inc.

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