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Homework Help: Integration of a function.

  1. Nov 12, 2008 #1
    1. The problem statement, all variables and given/known data
    Hey guys, I have one question: how can I integrate the function f(x,y,z)=x + y + z over the region between the paraboloid 4-x^2-y^2 and the xy-plane?

    2. Relevant equations
    For the paraboloid region, I used polar coordinates and found the volume of the region to be 8pi. Now, I have to find the integration of the other function x+y+z in this.

    3. The attempt at a solution
    Alright, I tried using a triple integral to no avail (rcos([tex]\theta[/tex]) +rsin(theta) +z)r drd(theta) dz. I do not know the limits of integration (hardest part of the problem for me). Is there anyway to solve this with only a double integral? Or would I have to use cylindrical/polar whatever triple integration to solve it? Thanks..!
  2. jcsd
  3. Nov 12, 2008 #2


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    Science Advisor

    How could you find the volume if you don't know the limits of integration? If it was because you integrated
    [tex]\int\int (4- x^2- y^2) dA= \int\int (4- r^2) r dr d\theta[/itex]
    then you should be able to see that is the same as
    [tex]\int\int\int_0^{4-x^2-y} dz dA= \int\int\int_0^{4- r^2} r dzdrd\theta[/tex]
  4. Nov 12, 2008 #3
    ohh....but how is the triple integral from 0 to 4-x^2-y^2 ? I know it's the function, but graphically, I don't understand. Oh well, my homework was due 10 minutes ago and I just turned it in (leaving this question blank) lol. Thanks for the answer though!
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