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Showing that three points are collinear if their cross product is zero

  1. Oct 1, 2009 #1
    1. The problem statement, all variables and given/known data
    show that three points P,Q, and R are collinear if and only if vec PQ x vec PR = 0


    2. Relevant equations



    3. The attempt at a solution
    i understand this (i can see this visually in my head) but i just don't know how to say it mathematically...i know that the angle between them must be 0 or 180, but how do i show this????
     
  2. jcsd
  3. Oct 1, 2009 #2

    rock.freak667

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    Homework Helper

    axb=|a||b|sinθ

    so what is the angle between PQ and PR?
     
  4. Oct 1, 2009 #3
    thanks haha =)
     
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