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Homework Help: Find the image set under a function

  1. Nov 7, 2011 #1
    1. f(x) = 1/4 (x - 1)^2 - 9. What is the image set of the function f? Express in interval notation.

    g(x) = 1/4 (x - 1)^2 - 9. (1 ≤ x ≥ 7). Specify the domain and image set of the inverse function g^-1, and find it's rule.

    I really don't understand what happens in these questions. I have looked at numerous examples, the easy ones look like it's similar to finding the range, and the hard ones i just dont get. Can someone please break it down for me.
  2. jcsd
  3. Nov 7, 2011 #2


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    the question asks based on the allowable values of x (the domain) what values can the function x take (the image)

    for example say x and f are both defined on the real line, for any x, f can never produce values less than [itex]8\frac{3}{4}[/itex], why?
  4. Nov 7, 2011 #3


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    What does 1 ≤ x ≥ 7 mean ?
  5. Nov 7, 2011 #4


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    I suspect that grain1 meant to say that [itex]1\le x\le 7[/itex].

    The graph of g(x) is a parabola with vertex at x= 1, y= -9, opening upward. In particular, for x between 1 and 7, the graph is one side of the parabola so it is "one to one" and there is an inverse. You find that inverse by solving the equation [itex](1/4)(x-1)^2- 9= y[/itex] of x. That's easy- [itex](x- 1)^2= 4(y+ 9)[/itex] and you can just take the square root of both sides. Knowing that you want x> 1 tells you which sign to use.
  6. Nov 8, 2011 #5
    Thanks Lanedance...

    -8 3/4 is the y - intercept, does this mean my image will be (-8 3/4, ∞)?

    SammyS ... yes i meant to type 1≤x≤7.

    HallsofIvy... i thought the inverse was found by switching the x and y in the oringinal formula i.e y = 1/4 (x-1)^2 - 9 becomes x = 1/4 (y-1)^2 - 9 and then solve for y

    I think i am more confused now than i was.
  7. Nov 8, 2011 #6


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    yes that image is correct, however it is just a concidence x=-8 3/4 is the y - intercept, the important part is that it is the minimum of the function

    to find an inverse never change x & y, the just confuses things, if you have
    y= y(x) = f(x)

    see if you can solve for x as a function of y which gives you the inverse function
    x= x(y) = f^{-1}(y)

  8. Nov 8, 2011 #7
    Thank you again Lanedance, you are being very helpful.

    I thought the min was at the vertex.... i.e. -9?

    g(x) = 1/4 (x - 1)^2 - 9 (1 ≤ x ≤ 7)

    g[itex]^{-1}[/itex](y) = 1/4 (y - 1)^2 - 9
    x = 1/4 (y - 1)^2 - 9
    4x + 36 = (y - 1)^2
    √(4x + 36) = y - 1
    +2x + 6 + 1 = y
    2x + 7 = g[itex]^{-1}[/itex]

    Can you tell me what is next?

    can i write the above as my inverse and that will give me an image of (9, 21)?
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