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Identifying Perpendicular Lines in Geometric Shapes

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  • 0:05 Perpendicular Lines
  • 0:31 Squares & Rectangles
  • 1:44 Triangles & Trapezoids
  • 3:02 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

Do you know what a perpendicular line is? Do you know how to spot one in a geometric shape? This lesson will tell you how to spot perpendicular lines in squares, rectangles, triangles, and trapezoids.

Perpendicular Lines

Lines on a paper, lines on a road, lines of people in front of a store - lines are used and seen everywhere, including in the world of math!

Let's angle this line-based lesson the right way from the very start! We'll be taking a look at how to identify special types of lines here. They are called perpendicular lines.

Perpendicular lines are lines that are found at right angles to one another. A right angle is exactly 90 degrees.

Squares & Rectangles

Right then. Right angles can be found in all sorts of geometric shapes. Let's start with the square.


square right angle symbol


Actually, the square on the screen right now is the proper symbol for a right angle. So if you ever see it drawn next to a pair of lines like you see now, you'll know that those lines are at a right angle to one another, even if your protractor says otherwise! This means, by definition, those two lines are also perpendicular to one another!


square with four right angles


Let's look at the square. The square is made of four lines. Because each line is perpendicular to two other lines, a square has four right angles.

Line AB is perpendicular to BC. BC is perpendicular to CD as well as AB and so forth.

So, yes, to make things clear: a square has right angles, and a square drawn by two additional lines in the corner of a right angle is used to symbolize a right angle as well.


rectangle with four right angles


Rectangles also have right angles. Again, let's look at the rectangle on our screen. Like a square, a rectangle has lines that are perpendicular to two other lines. This means it also has four right angles.

Triangles & Trapezoids

When it comes to triangles, things aren't as easy. That's because rectangles and squares must have four right angles, but a triangle may not have any!

So how do we know if a triangle has a right angle, and therefore, has perpendicular lines?

Well, if you have a square next to two lines in a triangle or someone tells you the triangle is a right triangle, then you know two of its lines must be perpendicular to one another and they form a right angle.


right triangle perpendicular


The image you see now has a little square in the corner, and that tells us this is a right angle, a right triangle, and thus, lines AC and CB are perpendicular to one another.

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