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Homework Help: Vector Magnitude

  1. Nov 4, 2007 #1
    I am not sure how to do the following question(Attempts all failed):
    Two vectors A and B are given by A = 4i + 8j and B = 6i – 2j. The scalar product of A and a third vector C is –16. The scalar product of B and C is +18. The z component of C is 0. What is the magnitude of C?
    Last edited: Nov 5, 2007
  2. jcsd
  3. Nov 4, 2007 #2
    Well, we know that A = <4, 8>, and B = < 6, -2>. If we dot A with a vector C and use the definition of the scalar product, we have:

    [tex] A_xC_x + A_yC_y = -16 [/tex]

    Doing the same for B we have:

    [tex] B_xC_x + B_yC_y = 18 [/tex]

    Now we have two equations and two unknowns. Does this help?
  4. Nov 4, 2007 #3
    Thanks a lot:) This is really far more simple than what I am thinking of !
    The answer is 3.6.
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