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Vectors problem

  1. Jun 5, 2012 #1
    1. The problem statement, all variables and given/known data

    The following are all vectors:
    A = <2, 1, 1>
    B = <1, -2, 2>
    C = <3, -4, 2>

    Find the projection of (A + C) in the direction of B

    2. Relevant equations

    Dot product?


    3. The attempt at a solution

    I was not sure what the meant in this question.

    I added A and C and I got (A+C) = <5, -3, 3>

    Then I did (A+C)dot(B) and I got that equal to 16



    I was also thinking of dotting (A+C) with the unit vector of B?
     
  2. jcsd
  3. Jun 5, 2012 #2
    Dot product is exactly what you want to do. :smile:

    [tex]\vec{X}\cdot\vec{Y} = |X|\cdot|Y|cos\theta[/tex]

    You have the dot product, |X|, |Y|, and you need cosine of the angle :wink:
     
  4. Jun 5, 2012 #3

    So is the question basically asking "What is the angle between (A+C) and B?"
     
  5. Jun 5, 2012 #4
    No, it is asking you for the projection(component) of A+C on B. What is the component of a vector X on another vector Y when the angle between them is θ??
     
  6. Jun 5, 2012 #5
    ok, I see. Thanks for the help.
     
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