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Homework Help: 8 orthogonal projection innequality

  1. Oct 28, 2011 #1
    [itex]U=\{x=(x_{1},x_{2},x_{3},x_{4})\in R^{4}|x_{1}+x_{2}+x_{4}=0\}[/itex]
    is a subspace of [itex]R^{4}[/itex]
    [itex]v=(2,0,0,1)\in R^{4}[/itex]
    find [itex]u_{0}\in U[/itex] so [itex]||u_{0}-v||<||u-v||[/itex]
    how i tried:
    i know that the only [itex]u_{0}[/itex] for which this innequality will work
    is if it will be the orthogonal projection on U paralel to v
    i am not sure about the theory of finding it
    what to do next?
    Last edited by a moderator: Oct 28, 2011
  2. jcsd
  3. Oct 28, 2011 #2


    Staff: Mentor

    OK, this is a basis for U.
    You can think of U as being a plane through the origin in R4. It doesn't hurt to visualize this as a plane in three dimensions. U is actually a hyperplane, being a space of one dimension less than the space it's in, but it's more convenient to think of this problem as a plane in R3.

    Vector v is not in the "plane" (U), so the vector u0 that is closest to v will be the vector that is directly under v, and lying in U. In other words, u0 is the projection of v onto the "plane" U. Surely in what you're studying there are some examples of how to find the projection of one vector onto another vector or a vector onto a plane.
  4. Oct 29, 2011 #3
    actually there no examples
  5. Oct 29, 2011 #4


    Staff: Mentor

    Here's a drawing to help you out.

    The drawing shows the subspace U as a plane. Vector v sticks up out of the plane, starting from the origin in R4. Vector u0 lies in the plane, and its tail is also at the origin.

    Vector w is perpendicular to the plane.

    The three vectors shown form a triangle, so B]u[/B]0 + w = v.
    The triangle is a right triangle, so u0 [itex]\cdot[/itex] w = 0.
    Any vector in the plane U can be written as a linear combination of the vectors in the basis you found. What must be true of every vector in the plane in relation to a vector that is perpendicular to the plane?

    I was able to solve this problem after I drew the picture shown here.
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