# Volumes of Solids of Revolutions Help!

1. Jul 28, 2012

### jzsc

1. The problem statement, all variables and given/known data

Find the volume of the solid obtained by rotating the region bounded by the given curves about the specified line. Sketch the region, the solid, and a typical disk or washer.

y=x, y=0, x=2, x=4; about x=1

3. The attempt at a solution
one of the radii is x=y but im not sure how to find the other one and the interval? Please explain this one to me. Thanks!

2. Jul 28, 2012

### SammyS

Staff Emeritus
Hello jzsc. Welcome to PF !

Which are you using, the disk method or the washer method ? Until you decide that, you can't specify a radius.

3. Jul 29, 2012

### jzsc

thank you SammyS! I am using the washer's method

4. Jul 30, 2012

### e^(i Pi)+1=0

$2\Pi\int x f(x) dx$

The x is serving as the distance from the y-axis (the radius), so if you're rotating around x=1 rather than x=0....

(I don't know how to put the a & b above and below the integrand)

Last edited: Jul 30, 2012
5. Jul 30, 2012

### ehild

$2\Pi\int_a^b x f(x) dx$

ehild