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Homework Help: Given a transformation of the plane F(x,y) = (2x+y,x-2y), find F^-1.

  1. Apr 18, 2014 #1
    1. The problem statement, all variables and given/known data
    Given a transformation of the plane F(x,y) = (2x+y,x-2y), find F-1.

    2. Relevant equations
    Actually this exercise had an item (a) which I had to prove this is a transformation. So I proved this function is injective and surjective.

    I know F(x,y) = (u,v) IFF F-1(u,v) = (x,y).

    3. The attempt at a solution
    I did it, but I don't know if this is the real solution...

    F(x,y) = (u,v) IFF F-1(u,v) = (x,y).
    u = 2x + y
    v = x - 2y
    Solving this, I concluded that: x = (v+2u)/5 and y = (u-2v)/5
    Is this the solution? F-1 = ((v+2u)/5, (u-2v)/5) or do I have to do anything else?
  2. jcsd
  3. Apr 18, 2014 #2


    Staff: Mentor

    Not quite. You should write the inverse as F-1(u, v) = ((v+2u)/5, (u-2v)/5).

    You've done most of the work already. All that's left is to show that F-1(u, v) = (x, y). Replace u and v by what you have above, and use the formula for F-1 that you found.
    Last edited: Apr 18, 2014
  4. Apr 18, 2014 #3


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    Science Advisor

    Alternatively, your teacher may prefer functions written in "x" and "y". In that case, replace your "u" with "x" and "v" with y: [itex]F^{-1}(x, y)= ((y+ 2x)/5, (x- 2y)/5)[/itex].
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