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Thread: Volume of a solid (integration word problem)

  1. #1
    New Member
    Join Date
    Nov 2011

    Volume of a solid (integration word problem)

    Find the volume of a solid whose base is enclosed by the circle x^2+y^2 = 9 and whose cross sections taken perpendicular to the x-axis are as follows:
    a. Semicircles
    b. Equilateral triangles
    ill just let the equation of circle be in terms of x since the area sweep is parallel to the x axis

    Since the lower limit of y is zero then the radius will just be
    [TEX]y = \sqrt{9-x^2}[/TEX]

    Area of semicircle
    [TEX]A = \frac{1}{2}\pi{r}^2[/TEX]
    then Volume
    [TEX]V = \int_{-3}^{3}{A}dx[/TEX]
    [TEX]V = \int_{-3}^{3}{\frac{1}{2}\pi{r}^2}dx[/TEX]
    [TEX]V = \int_{-3}^{3}{\frac{1}{2}\pi(\sqrt{9-x^2})^2}dx[/TEX]
    [TEX]V = 18\pi[/TEX]

    The lower portion and upper portion is
    [TEX]y = \sqrt{9-x^2} - (-\sqrt{9-x^2})[/TEX]
    [TEX]s=y = 2\sqrt{9-x^2}[/TEX]
    Area of equilateral
    [TEX]A = \frac{(s)^2\sqrt{3}}{4}[/TEX]
    [TEX]V = \int_{-3}^{3}{A}dx[/TEX]
    [TEX]V = \int_{-3}^{3}{\frac{(2\sqrt{9-x^2})^2\sqrt{3}}{4}}dx[/TEX]
    [TEX]V = 62.35[/TEX]

    i know this is the wrong answer because its not in the multiple choice answers
    a. 66pi
    b. 70pi
    c. 68pi
    d. 72pi

    a. 16sqrt(3)
    b. 17sqrt(3)
    c. 18sqrt(3)
    d. 19sqrt(3)
    Last edited by ZyzzBrah; 12-21-2011 at 06:59 AM.

  2. #2
    Elite Member
    Join Date
    Sep 2005
    Part a, I do not think you're wrong. I get [tex]18\pi[/tex] as well.

    Your work and logic seems OK. Same with part b.

    The choices given appear to be multiples of your answers.

  3. #3
    Elite Member
    Join Date
    Apr 2005
    PA, USA
    Estimate with a sphere of radius 3.

    [TEX]\frac{1}{2}\cdot\frac{4}{3}\pi 3^{3} = 18\pi[/TEX]

    Okay, was that really an "estimate"?

    Really, ALWAYS find a way to verify your work. In your "a", substitite +/- 3 with +/- r and substitute 9 with r^2 and see what you produce. It should look familiar.
    Last edited by tkhunny; 12-21-2011 at 02:55 PM.


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