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Find volume. Cross-sections are isosceles right triangles. Why is my answer wrong? :/

  1. Jan 22, 2013 #1
    Hi, I'm still practicing how to find volume.

    1. My problem is this:

    "Find the volume of the solid described below:

    The base of the solid is the disk x^2 + y^2 ≤ 4. The cross-sections by planes perpendicular to the y-axis between y=-2 and y=2 are isosceles right triangles with one leg in the disk."



    2. I thought that since an isosceles right triangle is really just half of a square, I could appraoch this problem like every cross-section was a square and divide the volume in half at the end...

    Using this method I have:

    A = s^2

    A= (2√(4-y^2))^2

    A= 4(4-y^2)

    A= 16 - 4y^2

    This is the area if every cross-section was a square.


    V= 1/2 * 2 ∫0 to 2 of 16 - 4y^2


    (The 1/2 on the outside is there because we need to find half of the volume to end up with isosceles right triangle cross-sections. The 2 is there because I changed the limits from -2 to 2 instead to 0 to 2.)

    V= ∫0 to 2 of 16 - 4y^2

    = 16y - 4*(y^3/3) |0 to 2

    = 16(2) - 4*[(2)^3)/3]

    = 32 - 10.67

    = 21.33

    But this is the wrong answer... Why is it wrong, though?

    I avoided having to think about the cross-sections as triangles, because the area formula for that is:
    1/2(b*h)

    I am confused as to what to plug in for the b and h...
    I know for an isoscleses right triangle, h = 1/2b
    Would b = sqrt(-y^2 + 4)...?
    And so Area= 1/2[sqrt(-y^2 + 4) * (1/2(sqrt(-y^2 + 4)))]

    ...I really don't know here. That's why I avoided triangles!
    Please help!
    Thank you VERY much! :)
     
  2. jcsd
  3. Jan 22, 2013 #2

    HallsofIvy

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    Re: Find volume. Cross-sections are isosceles right triangles. Why is my answer wrong

    What makes you think the answer is wrong? Everything you have done is correct, although I would prefer you leave the answer as 32- 32/3= (96- 32)/3= 64/3, the exact answer, rather than 21.33, an approximate answer.
     
  4. Jan 22, 2013 #3

    haruspex

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    Re: Find volume. Cross-sections are isosceles right triangles. Why is my answer wrong

    Not quite. Think that through again.
     
  5. Jan 22, 2013 #4
    Re: Find volume. Cross-sections are isosceles right triangles. Why is my answer wrong

    @haruspex- I still don't see why that would be wrong...

    Wouldn't each side be (2*(√4 - y^2))^2 ~ a (√(4 - y^2)) for the top part of the circle and a (√(4 - y^2)) for the bottom part of the circle?
     
  6. Jan 22, 2013 #5

    haruspex

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    Re: Find volume. Cross-sections are isosceles right triangles. Why is my answer wrong

    Maybe I have it wrong, but I thought that the x value, √(4 - y^2), would be half a diagonal of the complete square.
     
  7. Jan 22, 2013 #6
    Re: Find volume. Cross-sections are isosceles right triangles. Why is my answer wrong

    Oh, okay. Thanks for responding! :)
     
  8. Jan 22, 2013 #7

    Dick

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    Re: Find volume. Cross-sections are isosceles right triangles. Why is my answer wrong

    Right. It said a leg of the isoceles triangle is in the disk. If you are dealing with an online grading system it might want the exact answer as a fraction instead of a decimal approximation as Halls pointed out.
     
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