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Question Video: Counting the Edges of 3D Shapes Mathematics • 1st Grade

How many straight edges does the shape have?


Video Transcript

How many straight edges does this shape have?

We can see a picture of a 3D or a solid shape here. Can you think of any objects that are the same as this shape? Looks the same as perhaps a block of wood or maybe a long brick. The name of this shape is a cuboid. We know that it’s a cuboid because it has rectangular faces. It has four of these long rectangular faces all the way around. And then, at each end is what looks like it might be a square face. But we can just call them rectangles because squares are a special type of rectangle anyway.

So, this shape, with rectangular faces along the way around, is a cuboid. But although it’s helpful for us to think about the faces of this shape, the question doesn’t ask us about its faces. We need to say how many edges this shape has. Do you remember what the edge of a 3D shape is? An edge of a 3D shape is the place where two of its faces meet. It’s where the faces touch. So, if we pick two of the faces on this particular cuboid, we’ve got this long rectangular face here and the square face on the end.

Can you see the place where these two faces meet? It’s along here. This is one of the edges of the cuboid. If you imagine this cuboid is a long cardboard box and you open it out and flatten it, we’d be able to see this edge. It would be one of the folds. It’s where two faces meet. Did you notice this edge was one of the black lines that makes up our picture? But unfortunately, because this is a picture of a solid shape, we can’t see all of the edges. Some of them are hidden around the back. Let’s try and visualize what our shape looks like around the back. And we’ll use dotted lines to draw in the extra edges.

Well, from what we know about cuboids, the face on the bottom of this shape is exactly the same as the face on the top. It’s going to be another rectangle. In a way, this cuboid doesn’t really have a top and a bottom because all we’d have to do is turn it upside down; it’d look exactly the same. So, let’s draw in our missing sides of this rectangle on the bottom of the shape, one at the side and one along the back. And the sides of our rectangular face become the edges of this 3D shape.

We’ve got one more edge that’s missing. Can you spot where it belongs? It’s round the back on the left here. And it shows us that the end of our cuboid, there’s a square face. Can you see now that drawing in these dotted lines really helps us to visualize the shape? We don’t need to imagine what it looks like around the back; we can see.

Now that we’ve drawn in all of the edges, let’s count them. There are four straight edges that go all the way around this square face on the right. Then, there are another four edges around the opposite end of the cuboid. And then, finally, there are four more much longer edges that run along the length of the cuboid. So, that’s three groups of four. We know that three times four equals 12. So, we can say that the number of edges that this shape has is 12.

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