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Homework Help: Average value of a function over involving triple integral

  1. Nov 13, 2011 #1
    1. The problem statement, all variables and given/known data
    We define average value of function over a solid to be f = 1/Volume int int int f(x,y,z) dV

    So find the average value of the function f(x,y,z) = x^2z+y^2z over the region enclosed by paraboloid z = 1-x^2-y^2 and the plane z = 0

    3. The attempt at a solution
    Actually, solving this (I don't think) is much of a problem. I get answer 1/12. But I can't intuitively understand what is going on when/if I convert to polar to solve this problem.

    For example, I convert z = 1 - x2 - y2 and get z = 1 - r2

    Why should integrating this region (multiplied by r) in the boundaries 0≤r≤1 and 0≤θ≤2*pi give the region under f(x,y,z) = x^2z+y^2z? How is the information f(x,y,z) = x^2z+y^2z incorporated into these boundaries? Or have I actually done this wrong?
  2. jcsd
  3. Nov 13, 2011 #2
    Oh! Wait! We're not trying to find the volume under f(x,y,z) = x^2z+y^2z, are we? Indeed, we want to find the volume of z = 1-r^2. Right?

    Okay, I think this makes intuitive sense.
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