Video: Identifying the Number of Edges of 3D Shapes

How many edges does the pyramid have?

01:45

Video Transcript

How many edges does the pyramid have?

We know this picture shows a pyramid. And it’s not just because we’re told so in the question. We know it because we know what makes up a pyramid. This particular type of pyramid has a square face as its base and then four triangular faces that all meet at a point or a vertex right at the top. Now, our question asks us, how many edges does the pyramid have? It doesn’t mention faces at all. So, what have the faces of this pyramid got to do with its edges? Well, we know that an edge of a 3D shape is the place where two faces meet. It’s where they touch, where they connect up with one another. And so, knowing what the faces of this pyramid are like can really help us work out how many edges it has.

Remember, we started off by saying that the face at the base of this pyramid is a square. That’s why on this picture we can count one, two, three, four edges around the base. And because we know that the faces around a pyramid are all triangles, they all go up to that point at the top. We know from each of the vertices of this square base, we’re going to have one more edge. And four vertices means one, two, three, four more edges. And four plus another four equals eight. We know that the edges of a 3D shape are those places where two faces meet. And by counting them, we found that the number of edges that this pyramid has is eight.

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