November 5th, 2012, 06:07 AM  #1 
Newbie Joined: Oct 2011 Posts: 27 Thanks: 0  Rotation of a circle
Hello People, I have to solve a question, most of it is done. It is about the function f(x)=Sqr(r^2x^2) d) for the restriction r<=x<=r , calculate the volume and compare it to common equations in your equation booklet. i have done that, it gives the normal equation for the volume of a sphere. e) now its restricting the xaxis with r<=x<=a. For the volume i get this answer: ((r^2)a(1/3)(a^3))(2/3)(r^3) BUT, it asks me for common equations again, from my booklet. but neither in the internet nor in my booklet im able to find anything similar. Additionally, it does not make any sense to me, what would this equation calculate? Is there anything or is this just a trick question? Thanks 
November 5th, 2012, 07:03 AM  #2 
Global Moderator Joined: Nov 2009 From: Northwest Arkansas Posts: 2,766 Thanks: 4  Re: Rotation of a circle
How are you computing the volume of a curve?? Are you in calculus, perhaps studying surfaces of revolution??? 
November 5th, 2012, 09:34 AM  #3 
Newbie Joined: Oct 2011 Posts: 27 Thanks: 0  Re: Rotation of a circle
using pi times the defined integral of the squared function. gives me the volume of the rotated circle.

November 5th, 2012, 11:45 AM  #4 
Senior Member Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,211 Thanks: 521 Math Focus: Calculus/ODEs  Re: Rotation of a circle
We should get: where Now if we define: we have: 

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