# proving parallel lines with supplementary angles

They're just complementing each other. If two lines and a transversal form same-side interior angles that are supplementary, then the two lines are parallel. You know that the railroad tracks are parallel; otherwise, the train wouldn't be able to run on them without tipping over. Alternate Interior Angles Converse Another important theorem you derived in the last lesson was that when parallel lines are cut by a transversal, the alternate interior angles formed will be congruent. You have supplementary angles. With reference to the diagram above: ∠ a = ∠ d ∠ b = ∠ c; Proof of alternate exterior angles theorem. Those should have been obvious, but did you catch these four other supplementary angles? Again, you need only check one pair of alternate interior angles! If two lines are cut by a transversal and the alternate interior angles are equal (or congruent), then the two lines are parallel. Learn about one of the world's oldest and most popular religions. FEN Learning is part of Sandbox Networks, a digital learning company that operates education services and products for the 21st century. If two lines are cut by a transversal and corresponding angles are congruent, then the lines are parallel. Home » Mathematics; Proving Alternate Interior Angles are Congruent (the same) The Alternate Interior Angles Theorem states that If two parallel straight lines are intersected by a third straight line (transversal), then the angles inside (between) the parallel lines, on opposite sides of the transversal are congruent (identical).. This was the BEST proof activity for my Geometry students! Geometry: Parallel Lines and Supplementary Angles, Using Parallelism to Prove Perpendicularity, Geometry: Relationships Proving Lines Are Parallel, Saying "Happy New Year!" Figure 10.6l ‌ ‌ m cut by a transversal t. Excerpted from The Complete Idiot's Guide to Geometry © 2004 by Denise Szecsei, Ph.D.. All rights reserved including the right of reproduction in whole or in part in any form. Prove: ∠2 and ∠3 are supplementary angles. Angles in Parallel Lines. They cannot by definition be on the same side of the transversal. When a pair of parallel lines is cut with another line known as an intersecting transversal, it creates pairs of angles with special properties. For example, to say line JI is parallel to line NX, we write: If you have ever stood on unused railroad tracks and wondered why they seem to meet at a point far away, you have experienced parallel lines (and perspective!). Exam questions are included as an extension task. 9th - 12th grade. I'll give formal statements for both theorems, and write out the formal proof for the first. This is illustrated in the image below: Note that β and γ are also supplementary, since they form interior angles of parallel lines on the same side of the transversal T (from Same Side Interior Angles Theorem). If a transversal cuts across two lines to form two congruent, corresponding angles, then the two lines are parallel. I know it's a little hard to remember sometimes. Proving Lines are Parallel Students learn the converse of the parallel line postulate. These two interior angles are supplementary angles. 90 degrees is complementary. We are interested in the Alternate Interior Angle Converse Theorem: So, in our drawing, if ∠D is congruent to ∠J, lines MA and ZE are parallel. Whenever two parallel lines are cut by a transversal, an interesting relationship exists between the two interior angles on the same side of the transversal. Let's label the angles, using letters we have not used already: These eight angles in parallel lines are: Every one of these has a postulate or theorem that can be used to prove the two lines MA and ZE are parallel. If two lines are cut by a transversal and the consecutive exterior angles are supplementary, then the two lines are parallel. If two parallel lines are cut by a transversal, the interior angles on the same side of the transversal are supplementary. By reading this lesson, studying the drawings and watching the video, you will be able to: Get better grades with tutoring from top-rated private tutors. Let the fun begin. Mathematics. A transversal line is a straight line that intersects one or more lines. a year ago. Consider the diagram above. After careful study, you have now learned how to identify and know parallel lines, find examples of them in real life, construct a transversal, and state the several kinds of angles created when a transversal crosses parallel lines. Alternate Interior. Use with Angles Formed by Parallel Lines and Transversals Use appropriate tools strategically. answer choices . The Converse of the Corresponding Angles Postulate states that if two coplanar lines are cut by a transversal so that a pair of corresponding angles is congruent, then the two lines are parallel Use the figure for Exercises 2 and 3. I will be doing this activity every year when I teach Parallel Lines cut by a transversal to my Geometry students. Picture a railroad track and a road crossing the tracks. Theorem: If two lines are cut by a transversal and the interior angles on the same side of the transversal are supplementary, the lines are parallel. We've got you covered with our map collection. (This is the four-angle version.) Love! Infoplease knows the value of having sources you can trust. Let's go over each of them. How to Find the Area of a Regular Polygon, Cuboid: Definition, Shape, Area, & Properties. A similar claim can be made for the pair of exterior angles on the same side of the transversal. Which pair of angles must be supplementary so that r is parallel to s? Can you find another pair of alternate exterior angles and another pair of alternate interior angles? There are many different approaches to this problem. Exterior angles lie outside the open space between the two lines suspected to be parallel. Figure 10.6 illustrates the ideas involved in proving this theorem. To use geometric shorthand, we write the symbol for parallel lines as two tiny parallel lines, like this: ∥. Get help fast. Those angles are corresponding angles, alternate interior angles, alternate exterior angles, and supplementary angles. Need a reference? So, in our drawing, only … In our drawing, ∠B, ∠C, ∠K and ∠L are exterior angles. By using a transversal, we create eight angles which will help us. 348 times. The Same-Side Interior Angles Theorem states that if a transversal cuts two parallel lines, then the interior angles on the same side of the transversal are supplementary. Cannot be proved parallel. Lines MN and PQ are parallel because they have supplementary co-interior angles. If one angle at one intersection is the same as another angle in the same position in the other intersection, then the two lines must be parallel. These two interior angles are supplementary angles. Using those angles, you have learned many ways to prove that two lines are parallel. Proving Parallel Lines DRAFT. All the acute angles are congruent, all the obtuse angles are congruent, and each acute angle is supplementary to each obtuse angle. This geometry video tutorial explains how to prove parallel lines using two column proofs. Supplementary angles are ones that have a sum of 180°. Two lines are parallel if they never meet and are always the same distance apart. It's now time to prove the converse of these statements. You can use the following theorems to prove that lines are parallel. Infoplease is part of the FEN Learning family of educational and reference sites for parents, teachers and students. In our drawing, ∠B is an alternate exterior angle with ∠L. Around the World, ∠1 and ∠2 are supplementary angles, and m∠1 + m∠2 = 180º. Can you identify the four interior angles? If two lines are cut by a transversal and the alternate exterior angles are equal, then the two lines are parallel. Then you think about the importance of the transversal, the line that cuts across t… So, in our drawing, only these consecutive exterior angles are supplementary: Keep in mind you do not need to check every one of these 12 supplementary angles. Infoplease is a reference and learning site, combining the contents of an encyclopedia, a dictionary, an atlas and several almanacs loaded with facts. Let's split the work: I'll prove Theorem 10.10 and you'll take care of Theorem 10.11. Lines L1 and L2 are parallel as the corresponding angles are equal (120 o). Vertical Angles … (given) m∠2 = m∠7 m∠7 + m∠8 = 180° m∠2 + m∠8 = 180° (Substitution Property) ∠2 and ∠8 are supplementary (definition of supplementary angles) Not sure about the geography of the middle east? As promised, I will show you how to prove Theorem 10.4. 6 If you can show the following, then you can prove that the lines are parallel! A similar claim can be made for the pair of exterior angles on the same side of the transversal. Of course, there are also other angle relationships occurring when working with parallel lines. That should be enough to complete the proof. transversal intersects a pair of parallel lines. MCC9-12.G.CO.9 Prove theorems about lines and angles. line L and line M are parallel Proving that Two Lines are Parallel Converse of the Same-Side Interior Angles Postulate If two lines are cut by a transversal so that a pair of same-side interior angles are supplementary, then the lines are parallel. How can you prove two lines are actually parallel? Create a transversal using any existing pair of parallel lines, by using a straightedge to draw a transversal across the two lines, like this: Those eight angles can be sorted out into pairs. LESSON 3-3 Practice A Proving Lines Parallel 1. 5 Write the converse of this theorem. Supplementary angles add to 180°. In short, any two of the eight angles are either congruent or supplementary. Find a tutor locally or online. The last two supplementary angles are interior angle pairs, called consecutive interior angles. The second theorem will provide yet another opportunity for you to polish your formal proof writing skills. The diagram given below illustrates this. Consecutive exterior angles have to be on the same side of the transversal, and on the outside of the parallel lines. Proof: You will need to use the definition of supplementary angles, and you'll use Theorem 10.2: When two parallel lines are cut by a transversal, the alternate interior angles are congruent. This can be proven for every pair of corresponding angles … These four pairs are supplementary because the transversal creates identical intersections for both lines (only if the lines are parallel). And then if you add up to 180 degrees, you have supplementary. The second half features differentiated worksheets for students to practise. To prove two lines are parallel you need to look at the angles formed by a transversal. You could also only check ∠C and ∠K; if they are congruent, the lines are parallel. Both lines must be coplanar (in the same plane). ∠D is an alternate interior angle with ∠J. Consecutive exterior angles have to be on the same side of the transversal, and on the outside of the parallel lines. The Corresponding Angles Postulate states that parallel lines cut by a transversal yield congruent corresponding angles. Arrowheads show lines are parallel. Theorem: If two lines are perpendicular to the same line, then they are parallel. The converse theorem tells us that if a transversal intersects two lines and the interior angles on the same side of the transversal are supplementary, then the lines are parallel. We want the converse of that, or the same idea the other way around: To know if we have two corresponding angles that are congruent, we need to know what corresponding angles are. There are two theorems to state and prove. Vertical. Local and online. The first half of this lesson is a group/pair activity to allow students to discover the relationships between alternate, corresponding and supplementary angles. Which could be used to prove the lines are parallel? In our drawing, transversal OH sliced through lines MA and ZE, leaving behind eight angles. The previous four theorems about complementary and supplementary angles come in pairs: One of the theorems involves three segments or angles, and the other, which is based on the same idea, involves four segments or angles. Same-Side Interior Angles of Parallel Lines Theorem (SSAP) IF two lines are parallel, THEN the same side interior angles are supplementary. You can also purchase this book at Amazon.com and Barnes & Noble. Each slicing created an intersection. In our main drawing, can you find all 12 supplementary angles? As with all things in geometry, wiser, older geometricians have trod this ground before you and have shown the way. And if you have two supplementary angles that are adjacent so that they share a common side-- so let me draw that over here. Learn more about the mythic conflict between the Argives and the Trojans. But, how can you prove that they are parallel? When doing a proof, note whether the relevant part of the … Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc. To order this book direct from the publisher, visit the Penguin USA website or call 1-800-253-6476. Want to see the math tutors near you? Consecutive interior angles (co-interior) angles are supplementary. Let us check whether the given lines L1 and L2 are parallel. Just checking any one of them proves the two lines are parallel! Just like the exterior angles, the four interior angles have a theorem and converse of the theorem. You need only check one pair! 1-to-1 tailored lessons, flexible scheduling. converse alternate exterior angles theorem Which set of equations is enough information to prove that lines a and b are parallel lines cut by transversal f? When a transversal cuts across lines suspected of being parallel, you might think it only creates eight supplementary angles, because you doubled the number of lines. Learn faster with a math tutor. Therefore, since γ = 180 - α = 180 - β, we know that α = β. As you may suspect, if a converse Theorem exists for consecutive interior angles, it must also exist for consecutive exterior angles. 7 If < 7 ≅ <15 then m || n because ____________________. 68% average accuracy. Two angles are corresponding if they are in matching positions in both intersections. Two angles are said to be supplementary when the sum of the two angles is 180°. In our drawing, the corresponding angles are: Alternate angles as a group subdivide into alternate interior angles and alternate exterior angles. Proving that lines are parallel: All these theorems work in reverse. Brush up on your geography and finally learn what countries are in Eastern Europe with our maps. So if ∠B and ∠L are equal (or congruent), the lines are parallel. Proving Lines Are Parallel Whenever two parallel lines are cut by a transversal, an interesting relationship exists between the two interior angles on the same side of the transversal. Other parallel lines are all around you: A line cutting across another line is a transversal. (iii) Alternate exterior angles, or (iv) Supplementary angles Corresponding Angles Converse : If two lines are cut by a transversal so that corresponding angles are congruent, then the lines are parallel. Here are both pairs of alternate exterior angles: Here are both pairs of alternate interior angles: If just one of our two pairs of alternate exterior angles are equal, then the two lines are parallel, because of the Alternate Exterior Angle Converse Theorem, which says: Angles can be equal or congruent; you can replace the word "equal" in both theorems with "congruent" without affecting the theorem. If the two rails met, the train could not move forward. If we have two parallel lines and have a third line that crosses them as in the ficture below - the crossing line is called a transversal When a transversal intersects with two parallel lines eight angles are produced. Alternate exterior angle states that, the resulting alternate exterior angles are congruent when two parallel lines are cut by a transversal. If two lines are cut by a transversal and alternate interior angles are congruent, then the lines are parallel. And then we know that this angle, this angle and this last angle-- let's call it angle z-- we know that the sum of those interior angles of a triangle are going to be equal to 180 degrees. This means that a pair of co-interior angles (same side of the transversal and on the inside of the parallel lines… This is an especially useful theorem for proving lines are parallel. Given the information in the diagram, which theorem best justifies why lines j and k must be parallel? If two lines are cut by a transversal and the consecutive, Cite real-life examples of parallel lines, Identify and define corresponding angles, alternating interior and exterior angles, and supplementary angles. 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