# Homework Help: Shell Method Problem

1. Apr 10, 2012

### hvidales

1. The problem statement, all variables and given/known data
x=y^(2), x=4, about the x axis.

2. Relevant equations

2pi* integral from a to b of radius*height of function*thickness

3. The attempt at a solution
I have 2pi* integral from -2 to 2 of y*(4-y^(2)) dy but that does not make any sense. Answer comes out to be 0. The real answer is 8pi. I know how to do this with the disk method just not the shell method. Thank for the help!!

Last edited: Apr 10, 2012
2. Apr 10, 2012

### tiny-tim

hi hvidales!
noooo

3. Apr 10, 2012

### hvidales

I see and thanks. However, how come for this problem: y=x^(2), y=2-x^(2), about x=1 the limits are from -1 to 1 ?

4. Apr 10, 2012

### tiny-tim

you mean x goes from -1 to 1?

that's because the body in that case goes from x = -1 to x = 3 (the reflection about x = 1)

in your first example the body went from y = -2 to y = 2

in each case, you're taking half (because each single cylindrical shell is in both halves)

5. Apr 10, 2012

Thank you!!!