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Volume with triple integral

  1. Nov 21, 2011 #1
    1. The problem statement, all variables and given/known data

    Find the volume between the plane x+y+z = 1 and the xy-plane, for x+y[itex]\leq[/itex]2, x[itex]\geq[/itex]0, y[itex]\geq[/itex]0.

    3. The attempt at a solution

    First, the plane is above the xy-plane for y < 1-x and below the xy-plane for y > 1-x, so we'll need two integrals. This is how I set them up.

    [itex]\int[/itex][itex]^{1}_{0}[/itex][itex]\int[/itex][itex]^{1-x}_{0}[/itex][itex]\int[/itex][itex]^{1-x-y}_{0}[/itex]1dzdydx - [itex]\int[/itex][itex]^{2}_{0}[/itex][itex]\int[/itex][itex]^{2-x}_{1-x}[/itex][itex]\int[/itex][itex]^{1-x-y}_{0}[/itex]1dzdydx

    This gives me an answer of 7/6, but my book has 1 as the answer. I'm assuming my setup is wrong, since I checked the evaluation of the integrals in wolfram alpha. But I can't see what I did wrong. The innermost integrals give z-value of the given plane. The middle integrals sum those stacks from 0 to 1-x above the xy-plane and from 1-x to 2-x below the xy-plane. And then the outermost integrals sum those slices from the y axis to the boundary at 0=1-x and 0=2-x respectively.

    Help would be much appreciated.
  2. jcsd
  3. Nov 21, 2011 #2
    Weird. I tried evaluating your integrals, and I got (1/6)-(-3)=(19/6).

    See what you get by evaluating the second integral again. I don't know if I made a mistake or not.
  4. Nov 21, 2011 #3
    Try drawing a picture of the volume that you're trying to find. When I took multi-V, I noticed that helped me visualize the question better, which is essential to setting up your integral equation.
  5. Nov 21, 2011 #4


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    The dydx portion of your second integral needs to be broken into two sections. y goes from 1-x to 2-x only when x is between 0 and 1. When x is between 1 and 2 y goes from 0 to 2-x. Draw a picture of y = 1-x and y = 2-x in the xy plane and you will see it.
  6. Nov 21, 2011 #5
    Nice going.
  7. Nov 21, 2011 #6
    AHA! Thank you! I actually had already drawn the picture, but I've been staring at it forever without noticing that! Now I feel dumb. I'm kind of in a studying binge and I think I'm starting to burn out a bit. Thank you though.
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