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### Course: Class 6 (Foundation) > Unit 9

Lesson 4: Boxes and sketches# Intro to nets of polyhedra

Admittedly, "nets of polyhedra" sounds like the title of a bad sci-fi movie about man-eating, multi-headed fish. But in reality, nets of polyhedra are just 2D objects that wrap around 3D objects, like wrapping paper around presents. Created by Sal Khan.

## Want to join the conversation?

- wow these things are from like 5 years ago its 2020 now not 2015(29 votes)
- The top rated questions and answers appear first and these are generally the older ones. To see the more recent ones, change the sort order.(27 votes)

- I still question to this day how you draw and write on a computer so well. Especially with shapes.(25 votes)
- At5:15, He says that he isn't using a computer, he's hand-drawing.(8 votes)

- Is the net of 4D objects a bunch on 3D objects glued together?(8 votes)
**Yes**.

A hypercube is just like a cube, but it's 4-dimensional. A regular cube has a net made up of squares, but a hypercube has a net made up of cubes. Somehow, in the 4th dimension, you can fold cubes into cubes.

Hope this helps!(18 votes)

- Doesn't the base of the rectangular pyramid look like a square base?(8 votes)
- It does, but remember that:-

A square is a rectangle with all of its sides being equal, so in a way the base of a rectangular pyramid can possibly be in the shape of a square.(6 votes)

- who made the name Polyhedra(5 votes)
- Probably the Greeks. After all, the word comes from the Greek word Poluedros, meaning "many-sided".(8 votes)

- what about polygon where did it's name come from(6 votes)
- The word polygon has come from the convergence of two words

poly - many and gon - sides

therefore polygon is a many sided figure

hope I helped!(5 votes)

- So when he said that there are many other ways to open polyhedra up is that true?(6 votes)
- yes. a cube can be opened in a cross shape or a shape like this: _|-(2 votes)

- Admittedly, "nets of polyhedra" sounds like the title of a bad sci-fi movie about man-eating, multi-headed fish. But in reality, nets of polyhedra are just 2D objects that wrap around 3D objects, like wrapping paper around presents.(5 votes)
- what is a polyhedron? i know a polyhedra but not a polyhedron?(4 votes)
- The singular form of polyhedra.
*Polyhedra*would be the plural version of*polyhedron*(1 vote)

- does the net have to be exact for the square or other shape to be correctly folded(3 votes)
- yes, otherwise its not the faces of the polyhedrn(2 votes)

## Video transcript

- [Instructor] What we're
gonna explore in this video are polyhedra, polyhedra, which is just the plural of a polyhedron. And a polyhedron is a
three-dimensional shape that has flat surfaces and straight edges. So for example, a cube is a polyhedron. A cube is a polyhedron. All the surfaces are flat. All the surfaces are flat. And all of the edges, all of the edges are straight. So this right over here is a polyhedron. Once again, polyhedra is plural. Polyhedron is when you have one of them. This is a polyhedron. A rectangular pyramid is a polyhedron. So let me draw that. I'll make this one a little
bit more transparent. And lemme do this in a
different color just for fun. I'll make it a magenta,
rectangular pyramid. So once again, here I
have one flat surface. And then I'm gonna have four, four triangular flat surfaces. So this right over here, this is a rectangular pyramid. Now, it clearly looks like a pyramid. Why is it called a rectangular pyramid? Because the base right
over here is a rectangle. So these are just a few
examples of polyhedra. Now, what I wanna think
about are nets of polyhedron. Actually, let me draw and
make this transparent too. So we get full appreciation
of the entire polyhedron. Polyhedron, this entire cube. So now let's think about
nets of polyhedron, nets. So what is a net of a polyhedron? Well, one way to think about it is if you kind of viewed
this as made up of cardboard and you were to unfold it in some way so it would become flat. Or another way of thinking about it is if you were to cut out
some cardboard or some paper and you wanted to fold it up
into one of these figures, how would you go about doing it? And each of these polyhedra has multiple different
nets that you could create. So that can be folded up into
this three dimensional figure. So let's take an example. And maybe the simplest example
would be a cube like this. And I'm gonna color code it. So let's say that the bottom of this cube, the bottom of this cube
was this green color. And so I can represent it like this. That's the bottom of the cube. It's that green color. Now let's say that this
back surface of the cube is orange. Well, I could represent it like this. And notice I've kind of folded it out. I'm folding it out. And so if I were to flatten it out, it would look like this. Would look like that. Now, this other backside,
I'll shade it in yellow. This other backside right over here, I could fold it backwards and keep it connected along this edge. Keep it connected along this edge, fold it backwards, it would look like this. It would look like that. I think you get the general idea here. And just to be clear, this edge right over here is this edge right over there. Now I have to worry about this top part. The top part of the cube. Maybe it is in, lemme do it in a pink color. This top part of the cube
is in this pink color, and it needs to be attached
to one of these sides. I could attach it to
this side or this side. Let's attach it over here. So let's say it's attached to
that yellow side back here. So then when we fold it out, when we really unpack the thing, so we folded that yellow part back, then we're folding this part back, then it would be right over here. It would be right over there. And then we could fold, we could fold this front face, this front face right over here, we could fold that out
and this along this edge and it would go right over there. It would go right over there. And then we have one
face of the cube left. We have this side right over here. And we could, well, we could
do actually several things. We could fold it out along this edge and then we would draw, we would draw the
surface right over there. Or if we wanna do something interesting, we could fold it out. We could fold this out along the edge that it
shares with the yellow, that backside. So we could fold it out like this. So if we folded it out like this, it would be connected to the
yellow square right over here. So you see that there's many, many ways. There's many, many ways to construct a net or a net that when you fold it all back up will turn into this polyhedron, in this case, a cube. Let's do one more example. Let's do the rectangular pyramid 'cause all of these had rectangles, or in particular these had
squares as our surfaces. Now, the most obvious one might be to start with
your base right over here. Start with your base. And then take the different sides and then just fold them straight out. So for example, we could take this side right over here, fold it out, and it would look like, it would look like that. We could take this side back here. And once again, just fold it out. Fold it out, it'll look like that. It should be the same
size as that orange side, but I'm hand drawing it so
it's not gonna be perfect. So that's that right over there. And then you could take this front side right over here and once again, fold
it out along this edge so it would look like this. And then finally, you could take this side right over here and once again, fold
it out along this edge and it would go right there. But this isn't the only net
for this rectangular pyramid. There's other options, for example. And just to explore one of them, instead of folding that
green side out that way, instead, instead, we might have wanted to
fold it out along this edge, along this edge with the
yellow side that you can't see. Actually, let's make it
a little bit different. Let's fold it out along this
side since we can see the edge. And let me label, let me color the edge. So this is the edge right over here on the blue triangle. So this is the edge. And when you fold the green triangle out, it would look like this. If you fold the green triangle out, it would look like this. So hopefully this gives
you an appreciation. This gives you appreciation. There's multiple ways to unfold these three dimensional
figures, these polyhedra, or multiple ways to, if you wanted to do a cardboard cutout and then fold things back together to construct them. And these flattened versions
of them, these things, these unpacking of these polyhedra, we call nets.