# volume by shell; find my error

• Apr 6th 2010, 08:27 PM
Tclack
volume by shell; find my error
Find volume of shape between:$\displaystyle y= \sqrt{x}, y=0 , x=9$
So, since I'm going around vertically, I rearranged $\displaystyle x=y^2$

$\displaystyle \pi \int_0^3 (9-y^2)^2 dy$
$\displaystyle =\pi \int_0^3 (81-18y^2+y^4) dy$
$\displaystyle =\pi(81y-6y^2+\frac{y^5}{5})\mid_0^3$
$\displaystyle =\frac{1188}{5}\pi$

the answer is supposed to be 648pi/5

The similarities between my answer and their answer makes me think my method is correct. See attached picture. I'm trying to get the shape to be like a. But I suspect my equation is giving me b. Especially because my answer is larger, and b looks like it would have a larger volume.
Can anyone clarify?
• Apr 7th 2010, 03:10 AM
HallsofIvy
Quote:

Originally Posted by Tclack
Find volume of shape between:$\displaystyle y= \sqrt{x}, y=0 , x=9$
So, since I'm going around vertically, I rearranged $\displaystyle x=y^2$

So you are using disks not "shells".

Quote:

$\displaystyle \pi \int_0^3 (9-y^2)^2 dy$
$\displaystyle =\pi \int_0^3 (81-18y^2+y^4) dy$
$\displaystyle =\pi(81y-6y^2+\frac{y^5}{5})\mid_0^3$
You have a wrong power:
$\displaystyle =\pi(81y-6y^3+\frac{y^5}{5})\mid_0^3$

Quote:

$\displaystyle =\frac{1188}{5}\pi$

the answer is supposed to be 648pi/5

The similarities between my answer and their answer makes me think my method is correct. See attached picture. I'm trying to get the shape to be like a. But I suspect my equation is giving me b. Especially because my answer is larger, and b looks like it would have a larger volume.
Can anyone clarify?
• Apr 7th 2010, 04:02 PM
Tclack
Damn, thanks, I went over it 4 times and I didn't catch it. So I thought the error was in the setup of the integral.

Do the volumes of A and B equal each other?