Finding relative coordinates of a vector

  1. 1. The problem statement, all variables and given/known data

    I'm trying to find the coordinates of a vector [ 7 4 1 ] relative to these three vectors:
    [ -1 2 -9 ] , [ 1 5 1 ] , [ 47 -8 -7 ]

    2. Relevant equations

    none

    3. The attempt at a solution

    Do I get all these vectors into an augumented matrix?
    like:
    [ -1 1 47 | 7 ]
    [ 2 5 -8 | 4 ]
    [ -9 1 -7 | 1 ]
    and then row reduce completely? I got very awkward numbers and just wanted to make sure I'm using the correct method.
     
  2. jcsd
  3. HallsofIvy

    HallsofIvy 40,687
    Staff Emeritus
    Science Advisor

    I take it that "relative to these three vectors" means write it as a linear combination of those vectors. What you want are three numbers, a, b, c such that
    a<-1, 2, 9>+ b<1, 5, 1>+ c<47, -8, -7>= <7, 4, 1>. Looking at x, y, and z components separately that means -a+ b+ 47c= 7, 2a+ 5b- 8c= 4, 9a+ b- 7c= 1 which gives exactly the augmented matrix you have.

    Are you sure about that "47"? That number looks out of place.
     
  4. yes that 47 must be there because it was part of another question where I had to find a third vector orthogonal to the other two (involving the cross product) and I'm confident about that.

    If you say that my setup is right, then my more awkward numbers must be right.
    I got
    a = -4/43
    b = 28/27
    c = 145/1161
     
  5. oups, I made a mistake and it'll affect my a b c values. But thanks for confirming my method Ivy.
     
    Last edited: Jan 17, 2009
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