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Math Help - find the volume of the curves rotated about the x-axis

  1. #1
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    find the volume of the curves rotated about the x-axis

    find the volume of the curves rotated about the x-axis-4.png

    the endpoint comes -1 and 1

    and I am taking the integral of ((9/8)-x^2)-(x^2/8)....and then multiplyig by 2pi.

    the final answers come 3pi.

    Any help

    thanks
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  2. #2
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    method of washers ...

    \displaystyle V = \pi \int_a^b [R(x)]^2 - [r(x)]^2 \, dx
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    Quote Originally Posted by skeeter View Post
    method of washers ...

    \displaystyle V = \pi \int_a^b [R(x)]^2 - [r(x)]^2 \, dx
    so its like this with end points of -1 and 1
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  4. #4
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    Quote Originally Posted by racewithferrari View Post
    so its like this with end points of -1 and 1
    second term in the integrand should be \left(\frac{x^2}{8}\right)^2 if what you posted originally is correct ...

    \frac{x^2}{8} \ne \frac{1}{8x^2}

    also ... limits of integration?
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