Trapezoids: Definition and Properties

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  • 0:06 A Trapezoid
  • 0:40 Property
  • 1:02 Vocabulary
  • 2:03 Special Trapezoids
  • 3:03 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.

Watch this video lesson to learn what a trapezoid has to do with a triangle. Also, learn the special words that describe the various parts of a trapezoid. You will also see special types of trapezoids.

A Trapezoid

A trapezoid is a four-sided flat shape with one pair of opposite parallel sides. It looks like a triangle that had its top sliced off parallel to the bottom. Usually, the trapezoid will be sitting with the longest side down, and you will have two sloping sides for the edges. The top side is usually shorter than the bottom side. Doesn't it look like a triangle that had its top cut off?

Examples of trapezoids
examples of trapezoids

Try picturing all kinds of triangles and then cutting off their tops. You will be left with a bunch of trapezoids.


Trapezoids have one property that must be obeyed. The property is that it must have one pair of parallel sides. If you are looking at a trapezoid, you will see that it has two flat sides. These flat sides are the sides that are parallel to each other. If you extended these lines, they would never meet. Try it.


When working with trapezoids, there are a few words that we must add to our vocabulary.

The first word is bases, which are the sides that are parallel to each other. Picture a triangle and the bottom side will be one of your bases. The side that is created by slicing off the top of the triangle is the other base.

The second word to consider is legs. These are the sloping sides that form the left and right edge of a trapezoid that is sitting with the longest side down. Going back to the sliced triangle, the legs are the sides that go up and meet at the tip of the triangle. But, since the triangle has its top sliced off, the legs end where the slice occurred.

The third word is altitude, which is simply the height of the trapezoid. It is how tall the trapezoid is when sitting on a flat surface. You can find the altitude by measuring straight across from one base to the other.

Special Trapezoids

When your trapezoid's legs are the same length and when the angles each side forms with the bases are equal, then you have what is called an isosceles trapezoid. This means that, with the trapezoid sitting flat with the longest base down, the bottom two angles will be equal, and the top two angles will also be equal. Picture this kind of trapezoid as an isosceles triangle (a triangle with two equal sides and two equal angles) with its top cut off.

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