# bits n bobs

• Aug 22nd 2007, 06:17 AM
Amanda-UK
bits n bobs
few little grey areas:

1) how would i work this one out:

find the vloume generated by rotationg the area bounded by y=x^2 and y=4 around the line y=1
• Aug 22nd 2007, 07:38 AM
ThePerfectHacker
It is the same as rotating $\displaystyle (x+1)^2$ around $\displaystyle y=0$. Use the volume of rotation formula.
• Aug 22nd 2007, 08:44 AM
Amanda-UK
Quote:

Originally Posted by ThePerfectHacker
It is the same as rotating $\displaystyle (x+1)^2$ around $\displaystyle y=0$. Use the volume of rotation formula.

what formula is that?
x
• Aug 22nd 2007, 08:56 AM
Rebesques
$\displaystyle \int_a^b \pi f^2(x)dx$.
• Aug 22nd 2007, 09:04 AM
ThePerfectHacker
Quote:

Originally Posted by Rebesques
$\displaystyle \int_a^b \pi f^2(x)dx$.

It should be,
$\displaystyle 2\pi \int_a^b xf(x) dx$
• Aug 22nd 2007, 12:19 PM
Rebesques
I doubt it :eek:

This random page says otherwise:

Volume of A Solid of Revolution

Are you talking about surface area maybe?