- [Voiceover] Lets see if we can imagine
a three-dimensional shape
whose base could be viewed as
this shaded in region between
the graphs of Y is equal to F of X
and Y is equal G of X.
That's the base of our ...
this purple, this I guess mauve or purple color
is the base of it.
It's kind of popping out of our screen.
What I've drawn here in blue,
you could view this kind of the top ridge
of the figure.
If you were to take cross sections of the figure,
that's what this yellow line is.
If you were to take cross sections of this figure,
that our vertical, I should say
our perpendicular to the X axis,
those cross sections are going to be
isosceles right triangles.
This cross section
is going to look like this,
if you were going to flatten it out.
So over here it's sitting.
It's popping out of your page or out of your screen.
If you were to actually flatten it out,
the cross sections would look like this.
It's going to be an isosceles right triangle
with a hypotenuse of the isosceles right triangle
sits along the base.
Sits along the base.
So, it's isosceles. So that's equal to that.
It's a right triangle,
and then this distance
this distance between that point
and this point is the same as the distance
between F of X and G of X.
F of X and G of X for this X value right over there.
Now obviously that changes as we change our X value.
To help us visualize this shape here,
I've kind of drawn a picture of our coordinate plane.
If we've viewed as an angle,
if we're kind of above it,
you can kind of start to see
how this figure would look.
Once again, I've drawn the base.
I've drawn the base of it.
I've drawn the base of it right over there.
Maybe I should to make it clear.
Let me get like this.
Shade it in kind of parallel
to these cross sections.
So, I've drawn the base right over there.
There's two other sides.
There the side that's on ... I've drawn it here.
I guess you could view it on its top side
or the left side right over there.
Over on this picture that would be this.
When we're looking at it from above.
Then you have this other side.
I guess on this view
this one you could call kind of the right side.
Over here this is kind of the ...
when you viewed over here this is the bottom side.
The whole reason why I set this up,
and we tempting to visualize this figure.
I want to see if you can come up with a definite
integral that describes the volume of this figure.
That kind of almost looks like a football
if you cut it in half or a rugby ball.
It's skewed a little bit as well.
What's an expression a definite integral that
expresses the volume of this.
I encourage you to use the fact
that it intersects at the points.
These functions intersect at the point zero zero
and (c,d).
Can you come up with some expression
a definite in row of terms of zeros,
and Cs, and Ds, and Fs, and Gs
that describe the volume of this figure?
Assuming you've paused the video,
and have had a go at it, let's think about it.
If we want to find the volume,
one way to think about it is
we could take the volume of,
we could approximate the volume
as the volume of these individual triangles.
That would be the area of each of these triangles
times some very small depth.
Some very small depth.
I'll just shade it in to show the depth.
Some very small depth which we could call DX.
Once again we could find the volume of each of these
by finding the area,
the cross sectional area there,
and then multiplying that times
a little DX.
A little DX which would give us three,
so this is a little DX,
which would give us three-dimensional.
That's our DX. I could write that a little bit neater.
DX to give us a little bit of three-dimensional depth.
How could we ... what is the volume of
one of these figures going to look like?
Well, if we say, let me just call this height H.
We know that H is going to be F of X minus G of X.
That's this distance right over here.
Let's call that H.
We know that H is going to be,
maybe I should say H of X.
It is going to be a function of X.
H of X is going to be F of X.
F of X minus G of X. Minus G of X.
Given a H, what is going to be the area of this triangle?
This is a 45, 45, 90 triangle.
This is 90, then this is going to have to be 45 degrees.
That's going to have to be 45 degrees.
We know that the sides of a 45, 45, 90 triangle
or squared of two times the hypotenuse.
This is going to be squared of two over two
times the hypotenuse.
Squared two over two time the hypotenuse.
You could get that straight from the pythagorean theorem.
If the side, let's say it's length A,
then this side has length A.
You're going to have A squared plus A squared
is equal to the hypotenuse squared,
or two A squared is equal to the hypotenuse squared.
A squared is equal to the hypotenuse squared over two,
or that A is equal to H over the squared of two,
which is the same thing as a squared of two H over two.
I just rationalized the denominator,
multiplied by open numerator,
and then denominator by squared of two.
That's where I got this from.
What is the area going to be?
The area is just going to be your base times
your height times one-half.
Let me write that down.
The area
the area there
the area is just going to be the base,
which is squared of two over two times our hypotenuse,
times the height which is squared of two over two
time our hypotenuse, times one-half.
Times one-half.
If we didn't do that one-half,
we'd be figuring out the area of this entire square.
This is obviously where were concerned with
the triangle.
What's this going to be?
This is going to be squared of two over two
times squared of two over two is going to be
one-half and then you're going to have
another one-half.
So, it's one-fourth, one-fourth H square.
Did I do that right?
This is ... yes it's going to be two over four,
which is one-half, and then times another one-half.
This one-fourth H squared is the area.
Now, what's going to be the volume
of each of these triangles?
Well the volume of each of these triangles right over here,
the volume is just going to be our area times DX.
It's just going to be one-fourth H squared
times our depth. Times our depth DX.
If we just integrated a bunch of these,
from our X equals zero all the way to X equals C,
we essentially have our volume of the entire figure.
So how could we write that?
We want to write.
We want to essentially to find the volume of the figure.
We're kind of in the home stretch here.
Actually let me write volume.
This is volume of a section.
Volume of a cross section.
So, what's the volume of the entire thing going to be?
The volume
volume of the figure
of the figure
is going to be the definite integral,
the definite integral from X equals zero
to X equals C. X equals zero to C of one-fourth H squared.
One-fourth.
We know that H is equal to F of X minus G of X.
Instead of H, I'm going to write
F of X minus G of X squared
squared DX.
DX and we're done.
We just found an expression for,
a definite integral expression
I guess you could say,
for the volume of this strange figure
that we have defined.