We can add, subtract, multiply, and divide functions just as we can add subtract, multiply, and divide real numbers.
Operations with functions are defined using special notation.
Operation | Definition |
---|---|
Sum | $(f+g)(x)=f(x)+g(x)$(f+g)(x)=f(x)+g(x) |
Difference | $(f-g)(x)=f(x)-g(x)$(f−g)(x)=f(x)−g(x) |
Product | $(f\times g)(x)=f(x)\times g(x)$(f×g)(x)=f(x)×g(x) |
Quotient | $\left(\frac{f}{g}\right)(x)=\frac{f(x)}{g(x)}$(fg)(x)=f(x)g(x) where $g(x)\ne0$g(x)≠0 |
With each operation, the domain of the new function becomes the intersection or overlap of the domains of the original functions. The exception is that in the case of a quotient function, the new function's domain is further restricted to exclude values that make the denominator function zero.
Evaluate: Given $f(x)=\sqrt{x}$f(x)=√x and $g(x)=x^2-9$g(x)=x2−9, find $(f-g)(x)$(f−g)(x) and its domain.
Think: The domain of $f(x)$f(x) is $\left[0,\infty\right)$[0,∞) and the domain of $g(x)$g(x) is $\left(-\infty,\infty\right)$(−∞,∞). Therefore, the intersection of these sets is $\left[0,\infty\right)$[0,∞).
Do: Substitute and simplify. Then state the domain.
$(f-g)(x)$(f−g)(x) | $=$= | $f(x)-g(x)$f(x)−g(x) |
By definition |
$=$= | $\sqrt{x}-(x^2-9)$√x−(x2−9) |
Substitution |
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$=$= | $\sqrt{x}-x^2+9$√x−x2+9 |
Simplify |
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$D$D | $:$: | $\left[0,\infty\right)$[0,∞) |
The intersection of the two domains |
Evaluate: Given $f(x)=x+3$f(x)=x+3 and $g(x)=x^2-9$g(x)=x2−9, find $\left(\frac{f}{g}\right)(x)=\frac{f(x)}{g(x)}$(fg)(x)=f(x)g(x) and its domain.
Think: The domain of $f(x)$f(x) both and $g(x)$g(x) is $\left(-\infty,\infty\right)$(−∞,∞). However, since we are finding the quotient, we need to consider the case where $g(x)=0$g(x)=0 and remove it from the domain.
Do: Substitute and simplify. Then state the domain.
For the quotient we have:
$\left(\frac{f}{g}\right)(x)$(fg)(x) | $=$= | $\frac{f(x)}{g(x)}$f(x)g(x) |
By definition |
$=$= | $\frac{x+3}{x^2-9}$x+3x2−9 |
Substitution |
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$=$= | $\frac{x+3}{(x+3)(x-3)}$x+3(x+3)(x−3) |
Factor the denominator |
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$=$= | $\frac{1}{(x-3)}$1(x−3) |
Cancel common factors |
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$D$D | $:$: | $\left(-\infty,3\right)\cup\left(-3,3\right)\cup\left(3,\infty\right)$(−∞,3)∪(−3,3)∪(3,∞) |
The intersection of $D:f(x)$D:f(x), $D:g(x)$D:g(x), and where $g(x)\ne0$g(x)≠0. |
Given the following values:
$f\left(2\right)=4$f(2)=4, $f\left(7\right)=14$f(7)=14, $f\left(9\right)=18$f(9)=18, $f\left(8\right)=16$f(8)=16
$g\left(2\right)=8$g(2)=8, $g\left(7\right)=28$g(7)=28, $g\left(9\right)=36$g(9)=36, $g\left(8\right)=32$g(8)=32
Find $\left(f+g\right)$(f+g)$\left(2\right)$(2)
If $f(x)=3x-5$f(x)=3x−5 and $g(x)=5x+7$g(x)=5x+7, find each of the following:
$(f+g)(x)$(f+g)(x)
$(f+g)$(f+g)$\left(4\right)$(4)
$(f-g)(x)$(f−g)(x)
$(f-g)$(f−g)$\left(10\right)$(10)
Let $f\left(x\right)=\frac{9}{x-7}$f(x)=9x−7 and $g\left(x\right)=\sqrt{x-2}$g(x)=√x−2.
What is the domain of $f\left(x\right)$f(x)?
$($($-\infty$−∞$,$,$\infty$∞$)$)
$($($-\infty$−∞$,$,$0$0$)$)$\cup$∪$($($0$0$,$,$\infty$∞$)$)
$($($-\infty$−∞$,$,$9$9$)$)$\cup$∪$($($9$9$,$,$\infty$∞$)$)
$($($-\infty$−∞$,$,$7$7$)$)$\cup$∪$($($7$7$,$,$\infty$∞$)$)
What is the domain of $g\left(x\right)$g(x)?
$($($-\infty$−∞$,$,$\infty$∞$)$)
$($($-\infty$−∞$,$,$2$2$)$)$\cup$∪$($($2$2$,$,$\infty$∞$)$)
$[$[$2$2$,$,$\infty$∞$)$)
$($($2$2$,$,$\infty$∞$)$)
What is the domain of the function $(ff)(x)$(ff)(x)?
$($($-\infty$−∞$,$,$7$7$)$)$\cup$∪$($($7$7$,$,$\infty$∞$)$)
$[$[$2$2$,$,$\infty$∞$)$)
$($($-\infty$−∞$,$,$9$9$)$)$\cup$∪$($($9$9$,$,$\infty$∞$)$)
$($($-\infty$−∞$,$,$\infty$∞$)$)
Find the function $(ff)(x)$(ff)(x):
What is the domain of the function $(f/g)(x)$(f/g)(x)?
$($($-\infty$−∞$,$,$7$7$)$)$\cup$∪$($($7$7$,$,$\infty$∞$)$)
$($($2$2$,$,$7$7$)$)$\cup$∪$($($7$7$,$,$\infty$∞$)$)
$($($2$2$,$,$\infty$∞$)$)
$[$[$2$2$,$,$7$7$)$)$\cup$∪$($($7$7$,$,$\infty$∞$)$)
Find the function $(f/g)(x)$(f/g)(x):
A fifth way of combining functions exists that does not involve addition, subtraction, multiplication, or addition. Let's consider the meaning of a composition of functions through the following example.
Consider the function given by $f\left(x\right)=2x+1$f(x)=2x+1. We understand that the function takes values of $x$x in the domain and maps them to values $y=2x+1$y=2x+1 in the range.
Let's suppose, however, that this is only the first part of a two-stage treatment of $x$x. We now take these function values and map them using another function, say $g\left(x\right)=x^2$g(x)=x2. This means that the $y$y values given by $\left(2x+1\right)$(2x+1) become the squared values $\left(2x+1\right)^2$(2x+1)2. The diagram below captures the idea.
The output, or function values $f\left(x\right)$f(x) have become the input, or $x$x values of $g\left(x\right)$g(x). We can describe the complete two-stage process by the expression $g\left(f\left(x\right)\right)$g(f(x)). This is sometimes written as $(g\circ f)(x)$(g∘f)(x) and is called the composition of functions.
Algebraically, we can write $g\left(f\left(x\right)\right)=g\left[2x+1\right]=\left(2x+1\right)^2$g(f(x))=g[2x+1]=(2x+1)2.
Note that if we reversed the order of the two-stage processing, we would, in this instance, develop a different composite function. Here, $f\left(g\left(x\right)\right)=(f\circ g)(x)=f\left(x^2\right)=2\left(x^2\right)+1=2x^2+1$f(g(x))=(f∘g)(x)=f(x2)=2(x2)+1=2x2+1.
Using our understanding of function notation and evaluation, we are able to create and simplify the equations of composite functions as well as evaluate substitutions into them.
In a composition of functions, the inner function is evaluated first, followed by the outer function.
For example, in the composition $f(g(x))$f(g(x)) or $(f\circ g)(x)$(f∘g)(x), the function $g$g is applied first, followed by the function $f$f.
The domain of $(f\circ g)(x)$(f∘g)(x) is restricted to all $x$x-values in the domain of $g$g whose range values, $g(x)$g(x), are in the domain of $f$f.
Consider the functions $f\left(x\right)=-2x-3$f(x)=−2x−3 and $g\left(x\right)=-2x-6$g(x)=−2x−6.
Find $f\left(7\right)$f(7).
Hence, or otherwise, evaluate $g\left(f\left(7\right)\right)$g(f(7)).
Now find $g\left(7\right)$g(7).
Hence, evaluate $f\left(g\left(7\right)\right)$f(g(7)).
Is it true that $f\left(g\left(x\right)\right)=g\left(f\left(x\right)\right)$f(g(x))=g(f(x)) for all $x$x?
Yes
No
Consider the functions $f\left(x\right)=4x-6$f(x)=4x−6 and $g\left(x\right)=2x-1$g(x)=2x−1.
The function $r\left(x\right)$r(x) is defined as $r\left(x\right)=f\left(x^2\right)$r(x)=f(x2). Define $r\left(x\right)$r(x).
Using the results of the previous part, define $q\left(x\right)$q(x), which is $g\left(f\left(x^2\right)\right)$g(f(x2)).
Consider the functions $f\left(x\right)=x^2$f(x)=x2 and $g\left(x\right)=x+5$g(x)=x+5.
If $y$y is defined as $y=f\left(g\left(x\right)\right)$y=f(g(x)), state the equation for $y$y.
Graph the function $y$y.
What transformation of $f\left(x\right)$f(x) does $y$y correspond to?
A vertical translation $5$5 units up.
A horizontal translation $5$5 units to the left.
A horizontal translation $5$5 units to the right.
A vertical translation $5$5 units down.