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Homework Help: Another Vectors Problem

  1. Sep 13, 2009 #1
    1. The problem statement, all variables and given/known data
    For the following three vectors, what is -4.00 C·(4.00 A×B)?
    2. Relevant equations
    a x b = (a2b3 - a3b2) i + (a3b1 - a1b3) j + (a1b2 - a2b1) k

    3. The attempt at a solution
    I've multiplied 4 to each of the numbers in A, then plugged in numbers of A and B to the equation, next I multiplied -4 x C and used those to multiply the answer of A and B to get 224 + 1024 + 68k. I really am lost here.

    1. The problem statement, all variables and given/known data
    Use the definition of scalar product, vectors a ·b = ab cos θ, and the fact that vectors a · b= axbx + ayby + azbz to calculate the angle between the two vectors given by = 8.0i + 8.0j + 8.0k and = 7.0i + 5.0j + 6.0k.

    3. The attempt at a solution
    I found the magnitude of A = 11.3 and B = 8.6 using pythagorean theorem. Then I did 8x7 + 8x4 + 8x6 = 144. Then cos-1(144/97.2), but I don't get an angle...
    Last edited by a moderator: Apr 24, 2017
  2. jcsd
  3. Sep 14, 2009 #2


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    Science Advisor
    Homework Helper

    Hi Gattz! :smile:

    i] don't multiply the vectors by 4 at the start

    it makes the numbers too large :rolleyes:

    find C.(AxB) first, then multiply it by -16. :wink:
    ii] yes, this is correct … use it! (for AxB)

    (and don't bother with the (a1b2 - a2b1)k … you won't need it when you dot-product with C, will you? :smile:)
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