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How deserve to anyone be sure lines room parallel, if lines walk on forever? You and also your classmates might be brand-new to geometry, however geometry has existed for countless years, and also thousands of year ago, Euclid created down five postulates, one of which is the kernel of the Parallel Postulate.

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What"s a Postulate?

Euclid had many good ideas, yet not all can be proven. A postulate is one idea (also called an axiom) the is taken to be true also without proof. Comparison a postulate through a theorem, i beg your pardon is presented to be true by utilizing proofs.

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What are internal Angles?

Interior angles space the angles created when a transversal crosses two various other lines. The inner angles room between the two various other lines; exterior angles are outside the two other lines. Same-side internal angles are the 2 angles ~ above the very same side of the transversal.

What is the Parallel Postulate?

After Euclid knocked out four postulates (the structure for pure geometry), the waited prior to springing his 5th postulate, i beg your pardon in an English translate into by thomas Heath states:

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Parallel Postulate Definition


Parallel Postulate Example

The fastest method to understand the Parallel Postulate is to set up some line segments. Use uncooked spaghetti. Take two strands and also arrange castle a little apart from every other however leaning towards each other. Lay a third strand across the first two. You see you have created eight angle at the 2 intersections.

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Look in ~ the same-side interior angles towards the close end of spaghetti.

The sum of both same-side inner angles is less than 180°, for this reason Euclid is speak the lines stood for by the very first two spaghetti strands will, if extended, ultimately meet.

Draw The Parallel Postulate

Take a sheet of paper, pencil, and straightedge. Draw a quick line, possibly 10 cm long. Move away a few centimeters indigenous it and draw another 10 centimeter line. Now attract a transversal (line crossing both of those an initial two lines). If the two internal angles on the very same side include to much less than 180°, the attracted lines will, if lock continued, meet. Try it. Check out if they will certainly meet.

Now start again, yet this time, draw two parallel lines. When you draw in the transversal, the 2 same-side inner angles will certainly either be specifically 90° or will certainly be a mix of one acute and an obtuse angle.

For any type of line and a allude not on the line, Euclid shows us that only one line have the right to be constructed through that suggest that will certainly be parallel to the line. All other lines will ultimately intersect with that original line.

Euclid"s Parallel Postulate

Euclid"s Parallel Postulate permits that transversal come create many different angle as the cuts throughout the two lines, but it all boils down to just three possibilities:

The lines are not parallel and two same-side inner angles are less than 180°; the present will ultimately meet on the side of the transversal.

The lines room not parallel and two same-side interior angles are higher than 180°; the lines will never accomplish on the side the the transversal.

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The lines are parallel and any 2 same-side internal angles will be equal to 180°; the lines will never ever meet.

As long as the two interior angles top top the very same side of the transversal are much less than 180° (less than two ideal angles), the lines will certainly meet. That allows the transversal to also be in ~ a ideal angle to among the lines, with the other line creating an acute angle. Such a case will develop a triangle the the 2 lines and also their transversal, i beg your pardon connects straight to the Pythagorean Theorem.


No matter the mix of lines, transversals, and also same-side internal angles, Euclid"s Parallel Postulate stop true. Only in the special instance of parallel lines will certainly a transversal of any kind of angle produce four interior angles together that two same-side inner angles space equal come 180°.

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Lesson Summary

You have done a many work v some an extremely pure ideas. You deserve to now identify and also define a postulate, recall and also apply Euclid"s Parallel Postulate (his 5th postulate), and even test the Parallel Postulate.

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Types that Polygons


What friend learned:

Once you work your means through the video, drawings and also reading, friend will discover to:

Identify and define a postulate Recall and also apply the Parallel Postulate of Euclid check the Parallel Postulate