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Homework Help: Semi math question

  1. Aug 13, 2005 #1
    A(vec)=(2,1,-4) B(vec)=(-3,0,1) C(vec)=(-1,-1,2)

    whats the angle between Avec and Bvec in rads.

    i know the formula

    A(vec)*B(vec)= abs(A)*abs(B)*cos(theta)

    A(vec)*B(vec)=-10

    so why wont cos^-1(-10/(21)(10)) work?
     
  2. jcsd
  3. Aug 13, 2005 #2

    quasar987

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    absA = [itex]\sqrt{21}[/itex], not 21
     
  4. Aug 13, 2005 #3
    that still isnt right?
     
  5. Aug 13, 2005 #4

    HallsofIvy

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    What still isn't right?

    You were told that the length of vector A is [tex]\sqrt{21}[/tex], not 21.

    Did it then occur to you that the length of vector B should be [tex]\sqrt{10}[/tex], not 10?

    Try [tex]\theta= cos^{-1} \frac{-10}{\sqrt{210}}[/tex].
     
  6. Aug 13, 2005 #5
    no i realized that i just multiplied weird.. sorry thanks!
     
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