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Homework Help: F(x)=(x-1)(x+5) when f(1/x)

  1. Oct 16, 2006 #1
    I'm having difficulties in obtaining the book/calculator answer to the function
    f(x) = (x-1)(x+5) when f(1/x).

    The book answer is: [(1-a)(1+5a)]/a
    The answer I get is: 1/(a^2) + 4a - 5
    . or : (1 + 4a - 5a^2)/a^2
    . or : [(1 + 4a)/a^2] - 5

    I know that all these answers are possible answers, but I can't for the life of me figure out how the book got the answer (I've also checked with a graphing calculator, and recieved the same answers)

    - Thanks :)
  2. jcsd
  3. Oct 16, 2006 #2
    [tex] f(x) = (x-1)(x+5) [/tex]

    Then [tex] f(\frac{1}{a}) = (\frac{1}{a}-1)(\frac{1}{a} +5 )[/tex]

    Try working with the fractions (i.e. adding and subtracting) within the parentheses, then multiply them and see what you get.

    Also it shoud be [tex] \frac{(1-a)(1+5a)}{a^{2}} [/tex]
    Last edited: Oct 16, 2006
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