Graphing Composite Functions

• Sep 25th 2008, 10:03 AM
zodiacbrave
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
• Sep 28th 2008, 08:48 AM
Greengoblin
You're right for a composition $\displaystyle g(f(x))$ f's range is the domain of g....so for example, let:

$\displaystyle f:[-3,3]\rightarrow\mathbb{R}, f(x)=x+1$

$\displaystyle g:\mathbb{R}\rightarrow\mathbb{R}, g(x)=x^2$

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

So we have $\displaystyle f:[-3,3]\rightarrow[-2,4]$ and $\displaystyle g:[-2,4]\rightarrow[4,16]$ and for a compostion $\displaystyle f_1\circ f_2\circ ...\circ f_n(x)$ you need only know the domain of $\displaystyle f_n$ 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: $\displaystyle g\circ f(x) = (x+1)^2$ then choose a set of points from your domain e.g: $\displaystyle [-3,3]\in\mathbb{Z}$, 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.
• Sep 28th 2008, 12:52 PM
Moo
Quote:

Originally Posted by Greengoblin
You're right for a composition $\displaystyle g(f(x))$ f's range is the domain of g....so for example, let:

$\displaystyle f:[-3,3]\rightarrow\mathbb{R}, f(x)=x+1$

$\displaystyle g:\mathbb{R}\rightarrow\mathbb{R}, g(x)=x^2$

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

So we have $\displaystyle f:[-3,3]\rightarrow[-2,4]$ and $\displaystyle g:[-2,4]\rightarrow[4,16]$ and for a compostion $\displaystyle f_1\circ f_2\circ...\circ f_n(x)$ you need only know the domain of $\displaystyle f_1$ 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: $\displaystyle g\circ f(x) = (x+1)^2$ then choose a set of points from your domain e.g: $\displaystyle [-3,3]\in\mathbb{Z}$, 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 ;)
• Sep 29th 2008, 12:57 PM
Greengoblin
Thanks, its working now. hoorah! (Giggle)