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Math Help - geometry problem

  1. #1
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    geometry problem

    hey,

    Help is needed with the following geometry problem:

    Consider a triangle ABC. Insert a point D on the side Ac and a point E on the side AB. Draw a line through DE. The intersection between the line through DE and the line through BC is called F.

    Drawing an arbitrary triangle as mentioned above we can compute the following:

    |AD|*|CF|*|BE|
    |BF|*|AE|*|CD|

    It is possible to draw the triangle in a program called geogebra. By doing this we can move the points D and E and observe the change.

    The question is the following: formulate an expression which apparently holds true for the above-mentioned triangle.

    Thanks a bunch.
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  2. #2
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    Re: geometry problem

    I think you are referring to Menelaus' theorem, in that case, the ratio |AD|*|CF|*|BE| to |BF|*|AE|*|CD| is 1, i.e. they're equal.
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  3. #3
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    Re: geometry problem

    Thank you very much. Just one more question. I looked up the theorem on the Internet and sometimes I came across the ratio as being 1. why the differene?
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  4. #4
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    Re: geometry problem

    Some texts say 1 or -1, it's -1 if you treat them as vectors (e.g. AD = -DA). But if you're concerned only with magnitudes (e.g. |AD|), it's 1.
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