Main content

## 7th grade (Illustrative Mathematics)

### Unit 7: Lesson 8

Lesson 12: Volume of right prisms# Volume of triangular prism & cube

CCSS.Math: ,

Using the formulas for the volume of triangular prism and cube to solve some solid geometry problems. Created by Sal Khan.

## Video transcript

Let's do some solid
geometry volume problems. So they tell us, shown
is a triangular prism. And so there's a couple of types
of three-dimensional figures that deal with triangles. This is what a
triangular prism looks like, where it has a
triangle on one, two faces, and they're kind of separated. They kind of have
rectangles in between. The other types of triangular
three-dimensional figures is you might see pyramids. This would be a
rectangular pyramid, because it has a rectangular--
or it has a square base, just like that. You could also have
a triangular pyramid, which it's just literally
every side is a triangle. So stuff like that. But this over here is
a triangular prism. I don't want to get too much
into the shape classification. If the base of the
triangle b is equal to 7, the height of the
triangle h is equal to 3, and the length of the
prism l is equal to 4, what is the total
volume of the prism? So they're saying that
the base is equal to 7. So this base, this right
over here is equal to 7. The height of the
triangle is equal to 3. So this right over here, this
distance right over here, h, is equal to 3. And the length of the
prism is equal to 4. So I'm assuming it's
this dimension over here is equal to 4. So length is equal to 4. So in this situation, what
you really just have to do is figure out the area of
this triangle right over here. We could figure out the
area of this triangle and then multiply it by
how much you go deep, so multiply it by this length. So the volume is going to be
the area of this triangle-- let me do it in pink-- the
area of this triangle. We know that the
area of a triangle is 1/2 times the base
times the height. So this area right
over here is going to be 1/2 times the
base times the height. And then we're
going to multiply it by our depth of this
triangular prism. So we have a depth of 4. So then we're going to
multiply that times the 4, times this depth. And we get-- let's
see, 1/2 times 4 is 2. So these guys cancel out. You'll just have a 2. And then 2 times 3 is 6. 6 times 7 is 42. And it would be in some
type of cubic units. So if these were in-- I
don't know-- centimeters, it would be centimeters cubed. But they're not making us focus
on the units in this problem. Let's do another one. Shown is a cube. If each side is of
equal length x equals 3, what is the total
volume of the cube? So each side is equal length
x, which happens to equal 3. So this side is 3. This side over here,
x is equal to 3. Every side, x is equal to 3. So it's actually
the same exercise as the triangular prism. It's actually a
little bit easier when you're dealing with
the cube, where you really just want to find the area of
this surface right over here. Now, this is pretty
straightforward. This is just a
square, or it would be the base times the height. Or essentially the same,
it's just 3 times 3. So the volume is going to be the
area of this surface, 3 times 3, times the depth. And so we go 3 deep, so times 3. And so we get 3 times
3 times 3, which is 27. Or you might recognize
this from exponents. This is the same thing
as 3 to the third power. And that's why sometimes,
if you have something to the third power,
they'll say you cubed it. Because, literally, to
find the volume of a cube, you take the length of one side,
and you multiply that number by itself three times, one
for each dimension-- one for the length, the
width, and-- or I guess the height, the
length, and the depth, depending on how you
want to define them. So it's literally just
3 times 3 times 3.