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Linear Fractional Transformation - find the formula

  1. May 13, 2015 #1
    http://math.sfsu.edu/federico/Clase/Math350.S15/linea.JPG [Broken] 1. The problem statement, all variables and given/known data
    The picture below represents the map from a "green projective line" to a "red projective line." It takes the "green points" 1,3,7,-11 to the "red points" 0,6,10,20, respectively as shown by the ruler. Let f be the corresponding linear fractional, so f(1) = 0, f(3) = 6, f(7) = 10, f(-11) = 20. Find a formula for the "red point" f(x) on the ruler where the "green point" x lands.

    2. Relevant equations
    a linear fractional is where f(x) = (ax+b)/(cx+d) where ad-bc is not equal to 0.

    3. The attempt at a solution
    First a did a system of equations:
    (a(1) + b) / ( c(1) + d ) = 0
    (a(3) +b) / ( c(3) + d )= 6
    (a(7) +b) / ( c(7) + d ) = 10
    (a(-11) +b) / (c(-11) + d )= 20

    Thinking I could solve for a,b,c,d. However if I get all the letters in terms of let's say d and solve, this becomes nonsensical, since something d over something d is a number (the d's reduce) , and I'm left with a number = another number.
     
    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. May 13, 2015 #2

    Svein

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    Since you have a fraction, you can choose either a or c to be 1. So, choose c = 1. The first equation says a⋅1 + b = 0, so b = -a. That's two variables accounted for. Now the rest is easy...
     
  4. May 14, 2015 #3
    Choosing a or c to be 1 (or any number) seems to be an invalid assumption since even this is a ratio of a sum not just of a number over a number (choosing c=1 affects d, and choosing a=1 affects b), but please explain if I am incorrect. I chose c to be 1, then since I had everything else worked out in terms of d I plugged it all in. When I did, c=1 did not give f(3) = 6, so I solved for what c would be and got c = 1/6. Then, I tried to test this formula with f(7) = 10. It didn't work.
     
  5. May 14, 2015 #4

    Svein

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    I am sorry, but you have made an error somewhere. One solution is: a = 15, b= -15, c=1, d=2.

    About choosing one of the variables: A fundamental fact of fractions is that you can multiply or divide by the same number above or below. Therefore [itex] \frac{a\cdot x + b}{c\cdot x + d}=\frac{a(x+\frac{b}{a})}{a(\frac{c\cdot x}{a}+\frac{d}{a})}=\frac{c(\frac{a}{c}x+\frac{b}{c})}{c(x+\frac{d}{c})}[/itex] (of course, you cannot do that if the one you choose turns out to be 0).
     
  6. May 20, 2015 #5
    Thanks very much Svein.
     
  7. May 20, 2015 #6

    Mentallic

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    Another quick simplification is to notice the case where f(x)=0

    [tex]\frac{a+b}{c+d}=0[/tex]
    and for a fraction to equal 0, it means that the numerator = 0, hence

    [tex]a+b=0[/tex]

    [tex]a=-b\text{, or } b=-a[/tex]

    Therefore you just have an equation of the form

    [tex]f(x)=\frac{ax-a}{cx+d}[/tex]
     
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