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Multiplication using dot product.

  1. Oct 9, 2012 #1
    1. The problem statement, all variables and given/known data

    nxtje9.jpg

    2. Relevant equations

    Tan θ=A.B/|A|*|B|

    3. The attempt at a solution

    A=4i-9j
    B=9i-6j

    A.B=-18
    Magnitude of A = √97
    Magnitude of B = √117

    Solve that out using Cos-1 (-18/Sqr97*Sqr117) and I keep getting the wrong answer.
     
  2. jcsd
  3. Oct 9, 2012 #2

    gneill

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    Staff: Mentor

    Check the value you've calculated for A.B :wink:
     
  4. Oct 9, 2012 #3
    Ahh just caught that. Replugged in the values, and still came out wrong.

    Work:

    Plugged in 90/(Sqr97*Sqr117) = .8448190756

    Tan-1 (.84481....) = 40.191 degrees.

    Plugged that into online homework and it was wrong apparently.
     
  5. Oct 9, 2012 #4

    gneill

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    Staff: Mentor

    Check you definition of A.B in terms of magnitudes and angle. You used a correct method in the final line of your first post.
     
  6. Oct 9, 2012 #5
    Could you specify? Are you referring to A.B=|A||B|Cosθ?

    Edit: nevermind, got it!
     
  7. Oct 9, 2012 #6

    gneill

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    Staff: Mentor

    Yup. The formula involves a cosine, so why are you employing ##tan^{-1}##?
     
  8. Oct 9, 2012 #7
    Hahaha thanks, took me a while to catch that.. sadly.
     
  9. Oct 9, 2012 #8

    gneill

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    Staff: Mentor

    Glad I could help :smile:
     
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