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Help me figure out this f(x) problem, PLEASE! :)?

Given f(x) = 4x + 1 and g(x) = x² - 2, what is an expression for f(g(x))?

I have my ACT on Saturday, and I had this problem in a practice book. I couldn't figure it out...can you help me figure this out? Also, can you give me a simple step-by-step explanation on how to do these types of problems? Thanks!

P.S. The answer was 4x² - 7.

1 Answer

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  • 1 decade ago
    Favorite Answer

    Ok, let me explain functions to you.

    You've seen an equation like this many times:

    y=3x

    This equation can also be described as function y in variable x. Notation for this is y(x).

    If we solve for x instead to get x=y/3, this can be described as function x in variable y, or x(y).

    Now for function y, the original one, we can put anything in for a variable. Example:

    y(x) ---> y=3x

    y(z) ---> y=3z

    y(u+17) ---> y=3(u+17)

    y(5) ---> y=3(5)=15

    So for notation y(#), y is the function and # (which is whatever is in the parentheses) is what gets substituted for the variable.

    So for your problem above, there are actually 2 functions involved: f and g. In f(g(x)), g(x) is in the parentheses for f, so g(x) will be substituted in for the variable for f:

    f(g(x)) = f(x^2-2) = 4(x^2-2)+1 = 4x^2-7

    Read it over a couple times, you'll get it. Good luck :)

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