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Math Help - cylindrical shells

  1. #1
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    cylindrical shells

    i was working on this problem and im still not getting the right answer, can someone check my work and tell me where i went wrong? the formula is straight forward, i just dont see how im still not getting it right. the problem wants me to find the volume of the solid generated when the region bounded by xy = 4, x +y = 5 is revolved about the x - axis.IMG.pdf

    thanks in advance
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  2. #2
    Junior Member eddie2042's Avatar
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    Quote Originally Posted by slapmaxwell1 View Post
    i was working on this problem and im still not getting the right answer, can someone check my work and tell me where i went wrong? the formula is straight forward, i just dont see how im still not getting it right. the problem wants me to find the volume of the solid generated when the region bounded by xy = 4, x +y = 5 is revolved about the x - axis.IMG.pdf

    thanks in advance

    Your only mistake was that when you found the integral, you forget that

    \int5ydy = \frac{5y^2}{2}

    you forgot the denominator 2. Other than that you're fine. I tried it with the method of washers, using x as the main variable and got the same answer:

    V = 9\pi

    Cheers!
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  3. #3
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    thanks for that. the problem was worth 12 points and my instructor took off 4 pts for that mistake??? there are no shallow ends i suppose, so i must either sink or swim.....
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  4. #4
    Junior Member eddie2042's Avatar
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    Quote Originally Posted by slapmaxwell1 View Post
    thanks for that. the problem was worth 12 points and my instructor took off 4 pts for that mistake??? there are no shallow ends i suppose, so i must either sink or swim.....
    haha if you think that's crappy, you should look at my physics class. we have assignments that we have to hand in once per week. here's the marking scheme:

    You start off with 10/10.

    First Mistake: 8/10
    Second Mistake: RESUBMIT!!!

    lol ya.. universities don't like mistakes :P
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