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How deserve to you prove two lines room actually parallel? as with all points in geometry, wiser, larger geometricians have trod this ground before you and have displayed the way. By making use of a transversal, we produce eight angle which will assist us.

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What makes Lines Parallel?

Two lines are parallel if they never meet and are constantly the very same distance apart. Both lines have to be coplanar (in the same plane). To use geometric shorthand, we compose the symbol because that parallel lines as two tiny parallel lines, favor this: ∥. For example, come say heat JI is parallel to line NX, us write:



What room Parallel lines in real Life?

If friend have ever before stood top top unused rail tracks and wondered why they it seems ~ to meet at a point far away, you have experienced parallel present (and perspective!). If the two rails met, the train might not relocate forward. Various other parallel lines room all approximately you:

Street markingsCrosswalksBookshelvesNotebook paper

Parallel Lines cut By A Transversal

A heat cutting throughout another heat is a transversal. Once cutting across parallel lines, the transversal create eight angles. Develop a transversal using any type of existing pair of parallel lines, by making use of a straightedge to draw a transversal throughout the 2 lines, prefer this:


Proving Lines are Parallel

Those eight angles can be sorted out into pairs. Let"s label the angles, using letters we have actually not offered already:

Angles In Parallel Lines

These eight angles in parallel lines are:

Corresponding anglesAlternate interior anglesAlternate exterior anglesSupplementary angles

Every among these has actually a postulate or organize that have the right to be used to prove the two lines MA and ZE are parallel. Let"s walk over every of them.

Corresponding Angles

The Corresponding angle Postulate states that parallel lines reduced by a transversal productivity congruent equivalent angles. We desire the converse of that, or the same idea the other way around:

To understand if we have two equivalent angles that room congruent, we require to understand what corresponding angles are. In our drawing, transversal oh sliced through lines MA and also ZE, leave behind eight angles. Each slicing created an intersection.

If one edge at one intersection is the exact same as one more angle in the same position in the various other intersection, climate the 2 lines should be parallel. 2 angles are equivalent if they are in corresponding positions in both intersections.


In our drawing, the corresponding angles are:

∠B and ∠G

∠C and ∠J

∠F and ∠L

∠D and also ∠K

Alternate Angles

Alternate angles together a group subdivide into alternate interior angles and also alternate exterior angles. Exterior angle lie external the open space between the 2 lines suspected to it is in parallel. Interior angles lied within the open space between the two questioned lines.

In our drawing, ∠B, ∠C, ∠K and ∠L room exterior angles. Can you identify the four interior angles?

Did you to speak ∠D, ∠F, ∠G and ∠J?

Alternate angles appear on either next of the transversal. They cannot by meaning be ~ above the exact same side the the transversal. In our drawing, ∠B is an alternating exterior angle through ∠L. ∠D is an alternating interior angle v ∠J. Have the right to you find an additional pair of alternating exterior angles and also another pair of alternate interior angles?

Here space both pairs of alternating exterior angles:

∠B and also ∠L

∠C and also ∠K

Here room both bag of alternating interior angles:

∠D and ∠J

∠F and ∠G

Alternate Exterior Angles

If simply one of our 2 pairs of alternate exterior angles space equal, then the 2 lines room parallel, due to the fact that of the Alternate Exterior angle Converse Theorem, i beg your pardon says:


Angles deserve to be equal or congruent; you deserve to replace the word "equal" in both theorems through "congruent" without affecting the theorem.

So if ∠B and ∠L space equal (or congruent), the lines space parallel. Girlfriend could likewise only examine ∠C and also ∠K; if they space congruent, the lines room parallel. You need only inspect one pair!

Alternate interior Angles

Just like the exterior angles, the 4 interior angles have actually a theorem and also converse the the theorem. We space interested in the Alternate inner Angle Converse Theorem:


So, in ours drawing, if ∠D is congruent to ∠J, lines MA and ZE are parallel. Or, if ∠F is equal to ∠G, the lines room parallel. Again, you need only check one pair of alternate interior angles!

Supplementary Angles

Supplementary angles add to 180°. Supplementary angles produce straight lines, so once the transversal cuts throughout a line, the leaves 4 supplementary angles.


once a transversal cuts across lines doubt of gift parallel, you could think it just creates eight supplementary angles, since you doubled the number of lines.

Not true! it creates much more than eight!

In our main drawing, deserve to you uncover all 12 supplementary angles?

Around the top intersection:

∠Band∠C∠C and also ∠F∠F and ∠D∠D and ∠B

Around the bottom intersection:

∠G and also ∠J∠J and ∠L∠L and also ∠K∠K and also ∠G

Those should have actually been obvious, however did you catch these 4 other supplementary angles?

∠B and also ∠K∠L and also ∠C∠F and ∠J∠D and also ∠G

These 4 pairs space supplementary since the transversal creates identical intersections because that both lines (only if the lines are parallel). The last 2 supplementary angle are interior angle pairs, called consecutive internal angles.

Consecutive internal Angle Converse Theorem


As you might suspect, if a converse to organize exists because that consecutive inner angles, it must also exist because that consecutive exterior angles.

Consecutive Exterior edge Converse Theorem


Consecutive exterior angles need to be ~ above the exact same side of the transversal, and on the external of the parallel lines. So, in our drawing, just these continuous exterior angles are supplementary:

∠B and also ∠K

∠L and also ∠C

Keep in mind you perform not require to examine every among these 12 supplementary angles. Simply checking any type of one of them proves the two lines room parallel!

Lesson Summary

After careful study, you have actually now learned how to identify and also know parallel lines, uncover examples of lock in real life, construct a transversal, and state the numerous kinds the angles produced when a transversal crosses parallel lines.

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Those angle are corresponding angles, alternate interior angles, alternating exterior angles, and also supplementary angles. Making use of those angles, you have actually learned many ways to prove that 2 lines are parallel.

Next Lesson:

How to construct Parallel Lines

What friend learned:

By analysis this lesson, studying the drawings and also watching the video, you will certainly be maybe to:

Know what parallel present areCite real-life examples of parallel linesUse a transversalIdentify and also define corresponding angles, alternating interior and exterior angles, and supplementary anglesProve that two given lines are parallel