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Homework Help: Domain and Range (simple)

  1. Jan 21, 2010 #1
    1. The problem statement, all variables and given/known data

    Find the range for the function;

    fg(x) and state its range where

    [tex] f(x)= e^{2x} + 3[/tex] and [tex] g(x) = ln(x-1) [/tex]

    3. The attempt at a solution

    Putting g into f I reach;

    [tex] fg(x) = e^{2ln(x-1)}+3 [/tex] (1) - here is where I have the problem, now, I know that it simplifies down to

    [tex] fg(x) = (x-1)^2 + 3 [/tex] (2)

    However the question is, can [tex] x = 1 [/tex], it's obvious that for the function g, x > 1, or it's undefined, however when you put that into fg(x) the ln goes away so clearly in equation (2) fg >= 3, whereas with (1) fg > 3 (because x > 1). So basically I was wondering, it's obvious that g, f, & fg are linked, however is there a fundamental link between their domain/ranges? Or is it considered a new separate function?

    I'd appreciate some guidance, this was on an exam the other day.
     
  2. jcsd
  3. Jan 21, 2010 #2

    Mark44

    Staff: Mentor

    (f o g)(x) = f(g(x)) = e2ln(x - 1) + 3 = eln(x-1)2 + 3.

    Simplify the exponential term using the fact that eln(u) = u, for u > 0.

    On reading your post more carefully, I think you know this.

    The composite function f o g is tied to the two functions that make it up, but is different from both. You need to look at the domain of g, which will have a bearing on the domain of f o g. The
     
  4. Jan 21, 2010 #3
    Hmmm so fg(x) is not a function of x in its own right? it's dependant on the functions that make it up? That doesn't really make sense to me...=/

    As for the domain of g, I believe it's g > 0, but how does that help me?
     
  5. Jan 21, 2010 #4

    Mark44

    Staff: Mentor

    Yes f o g is a function in its own right. Note that when you write fg, it's usually interpreted to mean the product of two functions. IOW (fg)(x) = f(x)*g(x). What you're working with is the composition of two functions, which is why I have been writing the letter o between them. f o g (x) = f(g(x)).
    No that doesn't make any sense. g is the name of the function. For g(x) = ln(x - 1), the domain is {x| x > 1}. The input to the ln function has to be positive, so x - 1 > 0, so x > 1.
     
  6. Jan 21, 2010 #5
    Ah yeah, sorry, i did mean f(g(x)), my bad.

    So sorry for the confusion >.< but would the range therefore of f(g(x)) be >= 3? Sorry to be blunt but my teacher says it's just > 3 and i don't believe him >.<
     
  7. Jan 21, 2010 #6

    Mark44

    Staff: Mentor

    The range of f o g is {y | y > 3}. For the range to include 3, it would have to be true that eln(x - 1)2 is 0, which doesn't happen for any real number x.

    Think about it: eln(x - 1)2 is identically equal to (x - 1)2, for all x > 1. If x = 1, (x - 1)2 = 0, but eln(x - 1)2 is undefined.

    Also, x <= 1 is not in the domain of f o g, for the reason that x <= 1 is not in the domain of ln(x - 1).
     
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