When you were offered Postulate 10.1, you to be able come prove number of angle relationship that arisen when 2 parallel currently were reduced by a transversal. There space times when certain angle relationship are given to you, and also you require to identify whether or not the lines space parallel. You"ll construct some theorems to help you do this easily. Your very first theorem, organize 10.7, will be created using contradiction. The remainder of the theorems will certainly follow making use of a straight proof and also Theorem 10.7.

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

Let"s evaluation the steps connected in constructing a proof by contradiction. Begin by assuming the the conclusion is false, and also then mirroring that the hypotheses must additionally be false. In the initial statement of the proof, you start with congruent corresponding angles and conclude the the two lines space parallel. Come prove this theorem making use of contradiction, assume that the 2 lines are not parallel, and also show the the matching angles can not be congruent.

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

Theorem 10.7: If 2 lines are reduced by a transversal so the the equivalent angles room congruent, climate these lines room parallel.

A illustration of this case is presented in figure 10.8. 2 lines, l and m are reduced by a transversal t, and ?1 and also ?2 are equivalent angles.

Given: l and also m are reduced by a transversal t, together ?/? m. Prove: ?1 and also ?2 room not congruent (?1 ~/= ?2). Proof: Assume the l ?/? m. Due to the fact that l and also m are reduced by a transversal t, m and also t must intersect. You might contact the point of intersection of m and t the suggest O. Because l is not parallel to m, we can uncover a line, say r, that passes through O and also is parallel to l. I"ve attracted this brand-new line in number 10.9. In this new drawing, ?3 and ?2 are corresponding angles, so by Postulate 10.1, they are congruent. But wait a minute! If ?2 ~= ?3, and also m?3 + m?4 = m?1 through the Angle enhancement Postulate, m?2 + m?4 = m?1. Because m?4 > 0 (by the Protractor Postulate), this method that m?2 StatementsReasons
1. l and m room two lines cut by a transversal t, through 1 |/| mGiven
2. Let r be a heat passing with O i m sorry is parallel come lEuclid"s fifth postulate
3. ?3 and also ?2 are corresponding anglesDefinition of corresponding 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 of inequality
10. ?4 ~/= ?8 definition of ~=

That completes her proof by contradiction. The remainder of the theorems the you prove in this ar will manipulate Theorem 10.7. The remainder of the theorems in this ar are converses the theorems verified earlier.

Let"s take it a look in ~ some various other angle relationships that have the right to be provided to prove that two lines space parallel. These two theorems room similar, and to it is in fair I will certainly prove the first one and leave you to prove the second.

Theorem 10.8: If two lines are cut by a transversal so the the alternating interior angles space congruent, climate these lines room parallel. Theorem 10.9: If 2 lines are reduced by a transversal therefore that alternating exterior angles room congruent, climate these lines are parallel.

Figure 10.10 reflects two lines cut by a transversal t, with alternate interior angles labeled ?1 and ?2.

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

Given: l and m are cut by a transversal t, v ?4 ~= ?8. Prove: l ? ? m. Proof: The game plan is simple. In stimulate to use Theorem 10.7, you need to present that corresponding angles space congruent. You can use the reality that ?1 and also ?2 are vertical angles, for this reason they room congruent. Because ?2 and ?3 are corresponding angles, if you can display that they space congruent, climate you will have the ability to conclude the your lines room parallel. The transitive residential or commercial property of congruence will put the nail in the coffin, so come speak.StatementsReasons
1. l and also m are two lines cut by a transversal t, with ?4 ~= ?8 Given
2. ?1 and ?3 space vertical anglesDefinition of upright angles
3. ?1 ~= ?3 to organize 8.1
4. ?2 and ?3 are corresponding anglesDefinition of corresponding angles
5. ?2 ~= ?3 Transitive residential property of ~=
6. together ? ? mTheorem 10.7

Theorem 10.4 established the reality that if two parallel present are cut by a transversal, climate the inner angles top top the very same side the the transversal space supplementary angles. Theorem 10.5 claimed that if 2 parallel lines are reduced by a transversal, then the exterior angles on the very same side that the transversal are supplementary angles. It"s now time come prove the converse of this statements. Let"s split the work: I"ll prove theorem 10.10 and you"ll take treatment of theorem 10.11.

Theorem 10.10: If 2 lines are cut by a transversal so the the internal angles ~ above the very same side that the transversal are supplementary, then these lines space parallel. Theorem 10.11: If two lines are reduced by a transversal so the the exterior angles on the very same side of the transversal are supplementary, then these lines space parallel.

Figure 10.11 will aid you visualize this situation. Two lines, l and also m, are cut by a transversal t, with inner angles ~ above 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 also ?1 and ?2 are interior angles ~ above the same side that the transversal.

Given: l and m are cut by a transversal t, ?1 and ?2 room supplementary angles. Prove: together ? ? m. Proof: Here"s the game plan. In bespeak to usage Theorem 10.7, you need to show that corresponding angles space congruent. But it can be easier to usage Theorem 10.8 if friend can show that ?2 and also ?3 room congruent. You have the right to do that reasonably easily, if you use what friend discovered. Since ?1 and also ?3 are supplementary angles, and ?1 and ?2 space 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 also m room two lines reduced by a transversal t, ?1 and also ?2 space supplementary angles.Given
2.?1 and also ?3 are supplementary anglesDefinition of supplementary angles
3.?2 ~= ?3?1 and also ?3 room supplementary angles, and ?1 and ?2 space supplementary angles
4.l ? ? mTheorem 10.8

In a facility world, a complicated theorem requires a complicated drawing. If your drawing is also involved, it might be an overwhelming to decision which lines room parallel since of congruent angles. Take into consideration Figure 10.12. Expect that ?1 ~= ?3. Which lines have to be parallel? since ?1 and also ?3 are matching angles as soon as 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 i am through you in spirit and have listed the answers to these questions in prize Key.

If l ? ? m together in number 10.4, v m?2 = 2x - 45 and also m?1 = x, uncover m?6 and also m?8. Compose a official proof because that Theorem 10.3. Create a official proof for Theorem 10.5. Prove theorem 10.9. Prove theorem 10.11. In number 10.12, i m sorry lines must be parallel if ?3 ~= ?11 ?

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