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Homework Help: Dot Product help

  1. Jul 28, 2006 #1
    -Use the dot product to prove that the quadrilateral with vertices A(-4,1) B(4,5) C(7,-1) and D(-1,-5) is a rectangle.

    I tried find AB dot BC = 0, but I'm not getting 0. So I'm stumped. Can someone help me out? =/
  2. jcsd
  3. Jul 28, 2006 #2


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    Just check your arithmetic.
  4. Jul 28, 2006 #3
    Try BA dot BC
  5. Jul 28, 2006 #4


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    You are aware, are you not, that AB= -BA so that if AB dot BC= 0, so is BA dot BC?

    Hollysmoke, could you show us what you got for AB and BC? I wondering if you were sufficiently careful of signs while subtracting.
  6. Jul 28, 2006 #5
    pfft, you are right. That's what I get for posting late at night. :rolleyes:
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