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Homework Help: Solving a linear equation with a cross product

  1. May 14, 2012 #1
    1. The problem statement, all variables and given/known data

    Suppose v is a vector satisfying:

    [itex]\alpha v + ( a \times v ) = b[/itex]

    For [itex]\alpha[/itex] a scalar and [itex]a, b[/itex] fixed vectors. Use dot and cross product operations to solve the above for v.

    2. Relevant equations

    The unique solutions should be:

    [itex]v=\frac{\alpha^{2}b- \alpha (b \times a) + (b \cdot a)a}{\alpha(\alpha^{2}+|a|^{2}})[/itex]

    I'm having trouble getting there.

    3. The attempt at a solution

    I get that [itex]( a \times v ) = b - \alpha v[/itex] and then dotting both sides with a and v gives two identities:

    [itex](b - \alpha v) \cdot a = 0[/itex]
    [itex](b - \alpha v) \cdot v = 0[/itex]

    Which rearrange to give

    [itex](b \cdot a) = \alpha (v \cdot a)[/itex]
    [itex](b \cdot a) = \alpha (v \cdot v) = \alpha |v|^{2}[/itex]

    I don't know where to go from here. There's no inverse cross product and I've tried several different combinations of cross and dot products which lead to dead ends, or are just the same equations as those two I've just derived.

    Any help would be much appreciated.
  2. jcsd
  3. May 14, 2012 #2


    User Avatar
    Science Advisor
    Homework Helper

    Hi SP90! :smile:

    You have three vectors, a b and axb, with the third perpendicular to the other two.

    So start by finding the component of v along axb, and the component of v perpendicular to axb. :wink:
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