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Math Help - Graphing Composite Functions

  1. #1
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    Graphing Composite Functions

    Hello, I am given two graphics, F(x), G(x) and I am supposed to graph g(f(x))

    I am a bit rusty with composite functions, so I was hoping someone could give me some general information on how to do this? From what I recall, you need to take the range of f(x) and input it as the domain of g(x)? I know both functions domains must be satisfied. It looks like f(x) D = [-3,3] R = [-1,3] and g(x) D = [-3,3] R = [-2, 2]

    I hope this made sense, thank you
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  2. #2
    Member Greengoblin's Avatar
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    You're right for a composition g(f(x)) f's range is the domain of g....so for example, let:

     <br />
f:[-3,3]\rightarrow\mathbb{R}, f(x)=x+1<br />

     <br />
g:\mathbb{R}\rightarrow\mathbb{R}, g(x)=x^2<br />

    Then the range of f is [f(-3),f(3)] = [-2,4] then since g is composed with f, (also written g\circ f(x)) it's domain is the range of f, [-2,4]. Then the range of g is [g(-2),g(4)] = [(-2)^2,4^2] = [4,16]

    So we have f:[-3,3]\rightarrow[-2,4] and g:[-2,4]\rightarrow[4,16] and for a compostion f_1\circ f_2\circ ...\circ f_n(x) you need only know the domain of f_n to know all other domains/ranges.

    However, the best appraoch to graphing would be to write out the composition as a single expression, e.g: g\circ f(x) = (x+1)^2 then choose a set of points from your domain e.g: [-3,3]\in\mathbb{Z}, calculate a table of values, plot the points and interpolate them into a smooth curve...just as you would with the graph of a normal function.

    EDIT: Hmmm.. can someone please tell me the LaTeX for the composition symbol, the little circle? I thoubht it was: \circ
    ... I also tried \o and \O, but they didn't work either.
    Last edited by Greengoblin; September 29th 2008 at 01:58 PM. Reason: LaTeX errors
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  3. #3
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    Quote Originally Posted by Greengoblin View Post
    You're right for a composition g(f(x)) f's range is the domain of g....so for example, let:

     <br />
f:[-3,3]\rightarrow\mathbb{R}, f(x)=x+1<br />

     <br />
g:\mathbb{R}\rightarrow\mathbb{R}, g(x)=x^2<br />

    Then the range of f is [f(-3),f(3)] = [-2,4] then since g is composed with f, (also written g\circ f(x)) it's domain is the range of f, [-2,4]. Then the range of g is [g(-2),g(4)] = [(-2)^2,4^2] = [4,16]

    So we have f:[-3,3]\rightarrow[-2,4] and g:[-2,4]\rightarrow[4,16] and for a compostion f_1\circ f_2\circ...\circ f_n(x) you need only know the domain of f_1 to know all other domains/ranges.

    However, the best appraoch to graphing would be to write out the composition as a single expression, e.g: g\circ f(x) = (x+1)^2 then choose a set of points from your domain e.g: [-3,3]\in\mathbb{Z}, calculate a table of values, plot the points and interpolate them into a smooth curve...just as you would with the graph of a normal function.

    EDIT: Hmmm.. can someone please tell me the LaTeX for the composition symbol, the little circle? I thoubht it was: \circ
    ... I also tried \o and \O, but they didn't work either.
    Hi, yes it is \circ
    but if you don't put any space between \circ and f, it will be understood as \circf, which is an unknown command
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  4. #4
    Member Greengoblin's Avatar
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    Thanks, its working now. hoorah!
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