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Homework Help: Find an expression of g(x) in terms of x for an equation f(x)?

  1. Oct 13, 2009 #1
    1. The problem statement, all variables and given/known data
    The function f is defined by f(x) = x^3

    Find an expression for g(x) in terms of x in each of the following cases.

    (a) f [ g(x) ] = x+1

    (b) g [ f(x) ] = x+1

    2. Relevant equations

    3. The attempt at a solution

    I got the same answer both times. g(x) = cubic route (x+1). Because if I put the cubic route of x+1 in f(x), it is cubed, and I'm left with x+1.

    For (b) its the same. If I have cubic route of (x+1), and then cube it, I'm left with x+1. Is this correct?

    I'd appreciate some help :)
  2. jcsd
  3. Oct 13, 2009 #2
    The first one is okay. For the second one, if [tex] g(x) = \sqrt[3]{x + 1} [/tex] then [tex] g(f(x)) = \sqrt[3]{x^3 + 1} \neq x + 1 [/tex].

    Why don't you try [tex] g(x) = \sqrt[3]{x} + 1 [/tex]
  4. Oct 13, 2009 #3
    Thanks, that really clears things up.

    But is there a method that I can always apply to such a question?

    Or do you always have to keep doing trial and logic until you find the correct function?
  5. Oct 13, 2009 #4
    For the type of question you posted, it's more or less using your logic and just seeing what the correct function should be, combined with some trial and error.
  6. Oct 13, 2009 #5
    That's what I'm going to practice then. Thanks a million.
  7. Oct 13, 2009 #6


    Staff: Mentor

    For b, you want to find g so that g(x3 + 1) = x + 1, so you need to figure out what g needs to do to an input value so that the output is x + 1.

    To get from x3 + 1 to x + 1, g would need to:
    1. Subtract 1 from the input value.
    2. Take the cube root (not route) of the value from step 1.
    3. Add 1 to the value from step 2.
    JG89's suggestion, [tex] g(x) = \sqrt[3]{x} + 1 [/tex]
    doesn't do the first step, just the second and third, so doesn't work as the formula for g(x).
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