Main content

## Volumes with cross sections: triangles and semicircles

Current time:0:00Total duration:8:34

# Volume with cross sections: triangle

AP.CALC:

CHA‑5 (EU)

, CHA‑5.B (LO)

, CHA‑5.B.2 (EK)

## Video transcript

- [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.

AP® is a registered trademark of the College Board, which has not reviewed this resource.