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Math Help - Finding the volume of the graph

  1. #1
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    Finding the volume of the graph

    I'm supposed to find the volume of a graph. The equation of the graph is y=5-2cos(2x). The domain is from 0 to 3 pi/4. I have tried a million different ways, using integrals to figure this out, unfortunately, every single answer I get is wrong. I've tried u substitution as well as a few other methods, but I do not get a consistent area. I also don't understand how to get the volume from there, especially since if you rotate it, the radius wouldn't be constant.

    Answer choices are:
    a) 10pi+20.25pi^2
    b).5pi^2
    c).25
    d)1/9
    e)8pi + 21pi^2

    Help pretty please.

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  2. #2
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    Re: Finding the volume of the graph

    Quote Originally Posted by NothingButSmiles View Post
    I'm supposed to find the volume of a graph. The equation of the graph is y=5-2cos(2x). The domain is from 0 to 3 pi/4. I have tried a million different ways, using integrals to figure this out, unfortunately, every single answer I get is wrong. I've tried u substitution as well as a few other methods, but I do not get a consistent area. I also don't understand how to get the volume from there, especially since if you rotate it, the radius wouldn't be constant.

    Answer choices are:
    a) 10pi+20.25pi^2
    b).5pi^2
    c).25
    d)1/9
    e)8pi + 21pi^2

    Help pretty please.

    - NothingButSmiles
    What do you mean 'Volume' ....? Do you mean the volume of a solid of revolution? Around what axis? Please post the whole question so that all informtaion is given.
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    Re: Finding the volume of the graph

    Quote Originally Posted by mr fantastic View Post
    What do you mean 'Volume' ....? Do you mean the volume of a solid of revolution? Around what axis? Please post the whole question so that all informtaion is given.
    My paper says: Calculate the volume of the solid formed when the shaded region is rotated through 2pi radians about the x-axis.



    Then it gives a graph of y = 5 - 2 cos (2x) with a domain of 0 to 3pi/4
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    Re: Finding the volume of the graph

    Quote Originally Posted by NothingButSmiles View Post
    My paper says: Calculate the volume of the solid formed when the shaded region is rotated through 2pi radians about the x-axis.



    Then it gives a graph of y = 5 - 2 cos (2x) with a domain of 0 to 3pi/4
    Can you write down the integral you are required to solve?
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    Re: Finding the volume of the graph

    Quote Originally Posted by mr fantastic View Post
    Can you write down the integral you are required to solve?
    it doesn't say to use an integral, just to find the volume... I just used an itegral because that was what i thought I was supposeed to do
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  6. #6
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    Re: Finding the volume of the graph

    Can you describe the "shaded area"?
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  7. #7
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    Re: Finding the volume of the graph

    The correct answer is a). When the graph is rotated around the x axis it generates a volume. If you slice this volume with a blade perpendicular to the x axis and make a very thin slice, the slice is a disk whose radius is y or (5-2cos(2x)) the volume of the disk is y^2*pi *dx, where dx is the thickness of the disk. Now integrate this differential volume from 0 to 3pi/4, which gives you the whole volume. The result is: 10pi+20.25pi^2.
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    Re: Finding the volume of the graph

    Quote Originally Posted by NothingButSmiles View Post
    it doesn't say to use an integral, just to find the volume... I just used an itegral because that was what i thought I was supposeed to do
    Have you been taught the application of integration to calculating the 'volume of a solid of revolution'? If not, I'm not sure why you would be set the question.
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  9. #9
    Junior Member BERMES39's Avatar
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    Re: Finding the volume of the graph

    This may help a little better,
    Attached Thumbnails Attached Thumbnails Finding the volume of the graph-form-1.jpg  
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