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Homework Help: Finding unknown given vector a,b and parallel

  1. Feb 16, 2012 #1
    1. The problem statement, all variables and given/known data

    Given that c= 3i + 4j and d= i - 2j

    find μ when μc + d is parallel to i +3j

    2. Relevant equations

    3. The attempt at a solution

    3iμ + 4jμ + i + j

    i( 3μ + 1) + (4μ -2) j

    since it is parallel to i + 3j therefore 3μ+1=3(4μ-2)

    giving μ = 7/9

    However μ = -1

    Additional information:
    Apparently (see bold) I am supposed to multiply the LHS by 3 instead of the RHS but by equating I and J vectors and looking at the parallel line it would seem logical to do it the way I have (or am I being retarded?). I can accept that I have the multiply LHS instead of RHS but I don't understand why!!
  2. jcsd
  3. Feb 16, 2012 #2


    Staff: Mentor

    Not necessarily. Vectors can be parallel without being equal. Two vectors are parallel if either of them is some nonzero scalar multiple of the other.
  4. Feb 16, 2012 #3
    So i multiply the other side by 3 because it has to be that? ^
  5. Feb 16, 2012 #4


    User Avatar
    Science Advisor
    Homework Helper

    If a vector is parallel to i+3j, then 3 times the i component is equal to the j component. Since 3*1=3. So 3*(3μ+1)=(4μ-2). I'm not sure why you are doing it the other way around.
  6. Feb 16, 2012 #5
    Ok thanks! That kinda makes sense!

    actually nvm >.>
  7. Feb 16, 2012 #6
    I can understand why everyone hates vectors
  8. Feb 16, 2012 #7


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Vectors are marvelous !!!
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