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Homework Help: Help with what I think it's an impossible integral!

  1. Aug 27, 2012 #1
    So they give me the equation 3y2=x(1-x)2

    The idea is to find the surface area of the volume obtained by rotating around both axes.

    So let's start with a rotation around the x-axis, I decided to rewrite the equation as:

    I know that the surface area for a parallel rotation is S=∫2∏r√((dr/dx)2+1)

    and I know the derivative of the function, so I end up with:


    How the HELL am I supposed to integrate that?
  2. jcsd
  3. Aug 27, 2012 #2
    I think you made a mistake in determining the integrand. What you have for the dy/dx is not what I got when I differentiated the function (you also write the function as containing x(1-x)^2 initially and then when you rewrite it in terms of y you have an x(1-x^2) but I believe I got something different when I used either). Also, it doesn't look like you squared the dy/dx in the integrand. Finally, keep in mind that the r is equal to the value of the function, so you have to plug that in (You give the function in terms of y, but then you have r and dr/dx in the formula - those should be y and dy/dx... in this case, the radius is the height of the function)
  4. Aug 28, 2012 #3
    I can't even read that. Please consider typing up those integrals in LaTeX.
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