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Math Help - constructible lengths

  1. #1
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    constructible lengths

    Prove that if a real number, a, is constructible, then the real number (a/3) is also constructible.
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  2. #2
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    Quote Originally Posted by peprann3213 View Post
    Prove that if a real number, a, is constructible, then the real number (a/3) is also constructible.
    I am not really sure how you want a solution. I can show this by actually doing a construction. But the theorem says, let \mathbb{F} be the field of all constructable real numbers. Then it is an extension field over the rationals, \mathbb{Q}<\mathbb{F}<\mathbb{R}.

    Thus, the construcable field contains the rationals. Now since it is a field it is closed hence a/3\in \mathbb{F}
    Last edited by ThePerfectHacker; December 13th 2006 at 04:15 AM.
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  3. #3
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    Quote Originally Posted by peprann3213 View Post
    Prove that if a real number, a, is constructible, then the real number (a/3) is also constructible.


    There is nothing quite like giving the construction to prove something is
    constructible

    Draw a line, mark a segment of length a on the line.

    Draw a line not parallel to the original line through one end of the segment.

    Set the compass to some non-zero radius and mark a point at this radius
    from the point of intersection of the two lines on the second line. Now
    mark a second point at this radius from the previous point, and again mark a
    third point at this radius from the second.

    Draw a line from the third point to the other end of the original segment.

    Now draw a line parallel to this last line through the first point on the
    second line. then the segment from the point where the first two lines
    intersect to the point at which the parallel meets the first line is a segment
    of length a/3.

    (You should know how to draw a line parallel to a given line through any
    point not on the given line).

    RonL
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