# Double integrals 4 | Double and triple integrals | Multivariable Calculus | Khan Academy

I think it's very important to have as many ways as possible
to view a certain type of problem, so I want to introduce
you to a different way.
Some people might have taught this first, but the way I
taught it in the first integral video is kind of the way that I
always think about when I do the problems.
But sometimes, it's more useful to think about it the way I'm
about to show you, and maybe you won't see the difference,
or maybe you'll say, oh, Sal, those are just the
exact same thing.
Someone actually emailed me and told me that I should make it
so I can scroll things, and I said, oh, that's not
too hard to do.
So I just did that, and I scrolled my drawing.
But anyway, let's say we have a surface in 3 dimensions.
It's a function of x and y.
You give me a coordinate down here, and I'll tell you
how high the surface is at that point.
And we want to figure out the volume under that surface.
So.
We can very easily figure out the volume of a very small
column underneath the surface.
So this whole volume is what we're trying to figure out,
right, between the dotted lines.
I think you can see it.
You have some experience visualizing this right now.
So let's say that I have a little area here.
We could call that da.
Let me see if I can draw this.
Let's say we have a little area down here, a little
square in the x-y plane.
And it's, depending on how you view it, this side of it is dx,
its length is dx, and the height, you could say,
on that side, is dy.
Right?
Because it's a little small change in y there, and it's a
little small change in x here.
And its area, the area of this little square, is
going to be dx times dy.
And if we wanted to figure out the volume of the solid between
this little area and the surface, we could just multiply
this area times the function.
Right?
Because the height at this point is going to be the
value of the function, roughly, at this point.
Right?
This is going to be an approximation, and then we're
going to take an infinite sum.
I think you know where this is going.
But let me do that.
Let me at least draw the little column that I want to show you.
So that's one end of it, that's another end of it, that's the
front end of it, that's the other end of it.
So we have a little figure that looks something like that.
A little column, right?
It intersects the top of the surface.
And the volume of this column, not too difficult.
It's going to be this little area down here, which is,
we could call that da.
Sometimes written like that. da.
It's a little area.
And we're going to multiply that area times the height of
this column, and that's the function at that point.
So it's f of x and y.
And of course, we could have also written it as, this
da is just dx times dy, or dy times dx.
I'm going to write it in every different way.
So we could also have written this as f of
xy times dx times dy.
And of course, since multiplication is associative,
I could have also written it as f of xy times dy dx.
These are all equivalent, and these all represent the volume
of this column, that's the between this little area
here and the surface.
So now, if we wanted to figure out the volume of the entire
surface, we have a couple of things we could do.
We could add up all of the volumes in the x-direction,
between the lower x-bound and the upper x-bound, and then
we'd have kind of a thin sheet, although it will already have
some depth, because we're not adding up just the x's.
There's also a dy back there.
So we would have a volume of a figure that would extend from
the lower x all the way to the upper x, go back dy,
and come back here.
If we wanted to sum up all the dx's.
And if we wanted to do that, which expression would we use?
Well, we would be summing with respect to x first, so we could
use this expression, right?
And actually, we could write it here, but it
just becomes confusing.
If we're summing with respect to x, but we have the
dy written here first.
It's really not incorrect, but it just becomes a little
ambiguous, are we summing with respect to x or y.
But here, we could say, OK.
If we want to sum up all the dx's first, let's do that.
We're taking the sum with respect to x, and let me, I'm
going to write down the actual, normally I just write numbers
here, but I'm going to say, well, the lower bound here is x
is equal to a, and the upper bound here is x is equal to b.
And that'll give us the volume of, you could imagine a
sheet with depth, right?
The sheet is going to be parallel to the x-axis, right?
And then once we have that sheet, in my video, I think
that's the newspaper people trying to sell me something.
Anyway.
So once we have the sheet, I'll try to draw it here, too, I
don't want to get this picture too muddied up, but once we
have that sheet, then we can integrate those, we can
Because this width right here is still dy.
We could add up of all the different dy's, and we
would have the volume of the whole figure.
So once we take this sum, then we could take this sum.
Where y is going from it's bottom, which we said with c,
from y is equal to c to y's upper bound, to y
is equal to d.
Fair enough.
And then, once we evaluate this whole thing, we have the
volume of this solid, or the volume under the surface.
Now we could have gone the other way.
I know this gets a little bit messy, but I think
you get what I'm saying.
and getting this sheet, let's sum up the dy's first, right?
So we could take, we're summing in the y-direction first.
We would get a sheet that's parallel to the y-axis, now.
So the top of the sheet would look something like that.
So if we're coming the dy's first, we would take the sum,
we would take the integral with respect to y, and it would be,
the lower bound would be y is equal to c, and the upper
bound is y is equal to d.
And then we would have that sheet with a little depth, the
depth is dx, and then we could take the sum of all of those,
sorry, my throat is dry.
I just had a bunch of almonds to get power to be able
to record these videos.
But once I have one of these sheets, and if I want to sum up
all of the x's, then I could take the infinite sum of
infinitely small columns, or in this view, sheets, infinitely
small depths, and the lower bound is x is equal to a, and
the upper bound is x is equal to b.
And once again, I would have the volume of the figure.
And all I showed you here is that there's two ways of doing
the order of integration.
Now, another way of saying this, if this little original
square was da, and this is a shorthand that you'll see all
the time, especially in physics textbooks, is that we
are integrating along the domain, right?
Because the x-y plane here is our domain.
So we're going to do a double integral, a two-dimensional
integral, we're saying that the domain here is two-dimensional,
and we're going to take that over f of x and y times da.
And the reason why I want to show you this, is you see this
in physics books all the time.
I don't think it's a great thing to do.
Because it is a shorthand, and maybe it looks simpler, but for
me, whenever I see something that I don't know how to
compute or that's not obvious for me to know how to compute,
it actually is more confusing.
So I wanted to just show you that what you see in this
physics book, when someone writes this, it's the exact
same thing as this or this.
The da could either be dx times dy, or it could either be dy
times dx, and when they do this double integral over domain,
that's the same thing is just adding up all of these squares.
Where we do it here, we're very ordered about it, right?
We go in the x-direction, and then we add all of those up in
the y-direction, and we get the entire volume.
Or we could go the other way around.
When we say that we're just taking the double integral,
first of all, that tells us we're doing it in two
dimensions, over a domain, that leaves it a little bit
ambiguous in terms of how we're going to sum
up all of the da's.
And they do it intentionally in physics books, because you
don't have to do it using Cartesian coordinates,
using x's and y's.
You can do it in polar coordinates, you could do it
a ton of different ways.
But I just wanted to show you, this is another way to
having an intuition of the volume under a surface.
And these are the exact same things as this type of
notation that you might see in a physics book.
Sometimes they won't write a domain, sometimes they'd
write over a surface.
And we'll later do those integrals.
Here the surface is easy, it's a flat plane, but sometimes
it'll end up being a curve or something like that.
But anyway, I'm almost out of time.
I will see you in the next video.