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Geometry help

  1. Nov 6, 2005 #1
    The question is:
    Prove suppose A,B and C lie on a line L, with B between A and C, and that D is not on the line L. Prove that angle DBA + angle DBC = 180
    this question is obvious. But because its obvious its hard to prove.
    Say that X = angle DBA and Y = angle DBC
    therefor cos(X + Y) = -1 ... ive gotten this far, i just dont know how to obtain anymore information
    could i do cos(X) = u v/||u||||v|| and cos(Y) = v w/||v||||w||
    where v is the vector shared with the two angles..
    what do i do
  2. jcsd
  3. Nov 6, 2005 #2


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    Science Advisor
    Homework Helper

    I don't think this is going to help, but

    cos(X+Y) = cos(X)cos(Y)-sin(X)sin(Y)

    and you could get the sine from the cross products.

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