Constructing Perpendicular Lines in Geometry

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  • 0:05 Perpendicular Lines
  • 1:19 Setting It Up
  • 1:33 The Process
  • 2:52 Lesson Summary
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Lesson Transcript
Instructor: Yuanxin (Amy) Yang Alcocer

Amy has a master's degree in secondary education and has taught math at a public charter high school.

In this video lesson, you will see how to use geometric construction to draw perpendicular lines. Learn why geometric construction is useful. Also find out the only two tools in addition to your pencil that you need to do this.

Perpendicular Lines

Let's begin by talking about perpendicular lines. What exactly are they? Perpendicular lines can be defined as lines that intersect each other at 90 degree angles. Where do you see these in real life? Power poles are usually built perpendicular to the ground if the ground is flat and not on a hill. Also the window screens that keep bugs out are made with perpendicular lines. Take a close look at them and you will see tiny little squares and lots of perpendicular lines intersecting each other.

Why would you want to draw perpendicular lines? If you were an architect or an artist and you wanted to draw perpendicular lines to accurately portray something but you didn't have a way to measure angles, you would use geometric construction to draw your lines.

Geometric Construction

Geometric construction is the drawing of lines, shapes, and angles with only a pencil, compass, and straight edge. There are no numbers or any measuring involved at all. This is the method that you will learn in this video lesson. So if you have your own compass and straight edge, go ahead and grab them so you can follow along.

Setting It Up

To start, we are going to draw a line. Somewhere above the line we are going to draw a dot C. This point C is the point through which we are going to draw our perpendicular line.

The Process

Now take your compass and set one point on point C. Set the width of the compass so that the other end extends past your line. Now draw two arcs that intersect the line to the left and right of the point C. Label the intersection to the left point A and label the intersection to the right point B. You now have a line segment AB.

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