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Homework Help: Volume of Solid in R3

  1. Apr 18, 2010 #1
    Find the volume of the solid in the first octant of xyz space, bounded below by the coordinate axes and the unity circle and bounded above by z = 8xy.

    A) 1/2 B) 1 C) 2 D) 4 E) 8

    I know definitely volume will be the double integral of 8xy dy dx.

    I think my limits of integration for the inner integral should be -sqrt(1-x^2) to sqrt(1-x^2). Since we are looking at the unit circle ( x^2 + y^2 = 1)

    The outer integral limits should just be from -1 to 1?

    Is this correct?

    When I do the inner integral I get 4x(y^2) evaluated between -sqrt(1-x^2) to sqrt(1-x^2) but this looks like it just gives me 0 when I do the inner integral....

    4x(sqrt(1-x^2))^2 - 4x(-sqrt(1-x^2))^2

    Can someone help me and tell me if I am doing something wrong. I think i am
     
  2. jcsd
  3. Apr 18, 2010 #2

    radou

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    Your limits of integration are not correct.

    Edit: both "inner" and "outer". Recall that you're in an octant.
     
  4. Apr 18, 2010 #3
    oh right we are in the first octant.. so we are really only looking at a quarter of the unit circle i guess..

    So my limits on the outer integral should just be from 0 to 1 right? and the limits on the inner integral should be from 0 to (sqrt(1-x^2))/2? since its just a quarter of the circle? Thats my reason for dividing by 2
     
  5. Apr 19, 2010 #4

    radou

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    Yes, correct.
     
  6. Apr 19, 2010 #5
    change to the sphendrical coordinate
     
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