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Homework Help: Problem with vectors ?

  1. Mar 31, 2010 #1
    Heres the text :Find the angle of vectors a=3p+2q and b=p+5q and if p and q are orthogonal unit ? ... ?
    ok the formula is pretty simple cos=a*b/!a! * !b! =
    But once i progress and I get 3p square + 15pq + 2qp +10q square ,now I see here that 15pq and 2qp are eliminated is just BEEEYOOOND MEEE I MEAN hows that possible ,thanks in advance ?!?!?
  2. jcsd
  3. Mar 31, 2010 #2


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    Science Advisor

    p and q are orthogonal, which means (definition) that their dot product is 0.
  4. Mar 31, 2010 #3
    More specific please Im trying to understand for future references ,thnx
  5. Mar 31, 2010 #4
    Have you tried finding the vector sum of your two vectors? As the vector sum can be plugged into your formula, along with the two original vectors to find your angle, in which case you will need to do the inverse cos function. It.


    Where * is dotted on.

    Good luck.
  6. Apr 1, 2010 #5


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    Science Advisor

    Let a and b be arbitrary vectors (any dimension). The definition of orthogonal is the dot product is 0. Question for you - do you know what a dot product is?
  7. Apr 1, 2010 #6
    dot product !a! * !b! *cos theta

    But how in general can orthogonal plane have a dot product of 0 ?
  8. Apr 1, 2010 #7


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    Homework Helper

    If they are orthogonal, what is the angle between the vectors?
  9. Apr 1, 2010 #8
    like Zero ,lol ?
  10. Apr 1, 2010 #9


    Staff: Mentor

    Like, no.
    Do you know what orthogonal means?

    What you're asking here makes no sense.
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