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Math Help - Finding Inner and Outer Radius for Washer Method?

  1. #1
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    Finding Inner and Outer Radius for Washer Method?

    Hi, I have been having a tough time determining what the inner and outer radius are.
    Is the only way you can determine it is by looking at a graph to see the cone or cylindrical object?

    I have a graph bounded by certain regions, where it goes about the y-axis and x-axis, and some other parts of the graph.

    I wanted to make sure. About the y-axis means you take the shape you have bounded by the region and then copy the same image over the y-axis (so horizontally). While about x-axis means you copy the shape below it, right?

    The tough thing I have difficulty is determining what the inner and outer radius are. I tried looking through "Google," but I can only find where they just show which line matches each equation. I can see it from their diagrams when they point it out, but I have a tough time seeing it when I graph it out. Especially for inner radius and especially when I have graphs that show those "rectangles" on the line.

    Like http://17calculus.com/integrals/volu...plot-large.png
    Or http://17calculus.com/integrals/volu...plot-large.png

    I can't really picture the shape with those rectangles.
    Overall, I was wondering if someone else was able to explain it in a way I would be able to understand on finding the radius. I can do the Washer method just fine, but finding the outer and inner radius have been difficult.

    Thank you.
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  2. #2
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    Re: Finding Inner and Outer Radius for Washer Method?

    In the washer method, each "washer" is a rectangle rotated about the axis. So the inner radius is the edge of the rectangle close to the axis of rotation, and the outer radius is the edge of the rectangle away from the axis of rotation.

    It's really similar to finding the area between two curves - which curve gets the minus sign?

    I feel like I haven't explained it very well - does anyone else want to give it a try?

    - Hollywood
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  3. #3
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    Re: Finding Inner and Outer Radius for Washer Method?

    Quote Originally Posted by hollywood View Post
    In the washer method, each "washer" is a rectangle rotated about the axis. So the inner radius is the edge of the rectangle close to the axis of rotation, and the outer radius is the edge of the rectangle away from the axis of rotation.

    It's really similar to finding the area between two curves - which curve gets the minus sign?

    I feel like I haven't explained it very well - does anyone else want to give it a try?

    - Hollywood
    I think I understand. I'll try to apply that. Thanks
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