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Triple Integral

  1. Oct 26, 2013 #1
    1. Use a triple integral to find the volume of the given solid.

    The solid enclosed by the cylinder x^2 + z^2 = 4 and the planes y = -1 and y + z = 4


    This looked like a cylindrical coordinate system to me, except for the fact that it is not cylindrical around the z-axis but the y-axis. I tried to fix this problem by "rotating" my coordinate axes so that my old z-axis would be my new x-axis, x would become y, and y would become z. I'm not sure if this is a valid approach or not.
     
  2. jcsd
  3. Oct 26, 2013 #2

    hilbert2

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    It makes no difference mathematically how we name the coordinate axes. The coordinate system ##(r,\theta,y)##, with
    ##x=rcos\theta##
    ##z=rsin\theta##,
    is as valid a cylindrical coordinate system as the usual ##(r,\theta,z)##.

    Now the volume element is ##dxdydz=rdrd\theta dy##. Note that because of the plane ##y+z=4## all of the integration limits in the volume calculation are not constants, one of them is a function of ##r## and ##\theta##.
     
  4. Oct 26, 2013 #3
    Hilbert1 is absolutely correct. However, if you lack his sophistication, you can just rewrite the problem switching the y and z. They are just symbols, right?
     
  5. Oct 27, 2013 #4
    Thank you both so much!
     
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