Supplemental Videos
The main topics of this section are also presented in the following videos:
The main topics of this section are also presented in the following videos:
Recall that a function is a rule that takes inputs and assigns to each input exactly one output. We say the output is a function of the input.
Is the number of letters a function of a word?
First, identify what is the input and what is the output. You can write down the following table as an example.
Input: Word | Output: Number of Letters |
I | 1 |
Love | 4 |
Math | 4 |
Class | 5 |
In this case, the number of letters is a function of the word, since for each input (word) has exactly one output (number of letters).
When we do have a function we use a mathematical shortcut to talk about it. We write \(output = f(input)\) and we can use any letter to represent the function, the input or the output. For example, to indicate that a quantity \(y\) is a function of quantity \(x\text{,}\) we write \(y = f(x)\) and say \(y\) equals \(f\) of \(x\)
. Note: \(f(x)\) represents the output of the function \(f\text{,}\) when \(x\) is the input.
Recall that finding the value of the output variable that corresponds to a particular value of the input variable is called evaluating the function.
If a function is described by an equation, we substitute the given input value into the equation (replacing the input variable) to find the corresponding output, or function value.
Sometimes, you will see \(y=f(x)=3x+2\) (or some other expression on the right) which may cause discomfort since there's a lot happening. However, this is a helpful tool to clarify what label we're giving the outputs, in this case \(y\) is the output label. In the following example, you will see the use of \(H=f(s)\) and you can easily intepret the output as "H"eight with the input being "s"econds.
The function \(H\) is defined by \(H=f(s) = \dfrac{(s+3)^2}{s}\text{.}\) Evaluate the function at the following values.
\(s=6\)
\(s=-1\)
\(f(\alert{6})=\dfrac{(\alert{6}+3)^2}{\alert{6}}= \dfrac{9^2}{6}=\dfrac{81}{6}=\dfrac{27}{2}\text{.}\) Thus, \(f(6)=\dfrac{27}{2}\text{.}\)
\(f(\alert{-1})=\dfrac{(\alert{-1}+3)^2}{\alert{-1}}= \dfrac{2^2}{-1}=-4\text{.}\) Thus, \(f(-1)=-4\text{.}\)
Complete the table displaying ordered pairs for the function \(f(x) = 5 - x^3\text{.}\) Evaluate the function to find the corresponding \(f(x)\)-value for each value of \(x\text{.}\)
\(x\) | \(f(x)\) | |
\(-2\) | \(\) | \(f(\alert{-2})=5-(\alert{-2})^3=~\) |
\(0\) | \(\) | \(f(\alert{0})=5-\alert{0}^3=\) |
\(1\) | \(\) | \(f(\alert{1})=5-\alert{1}^3=\) |
\(3\) | \(\) | \(f(\alert{3})=5-\alert{3}^3=\) |
\(x\) | \(f(x)\) |
\(-2\) | \(\alert{13} \) |
\(0\) | \(\alert{5}\) |
\(1\) | \(\alert{4}\) |
\(3\) | \(\alert{-22}\) |
To simplify the notation, we sometimes use the same letter for the output variable and for the name of the function. In the next example, \(C\) is used in this way.
Sometimes we need to evaluate a function at an algebraic expression rather than at a specific number.
TrailGear manufactures backpacks at a cost of
for \(x\) backpacks. The company finds that the monthly demand for backpacks increases by 50% during the summer. The backpacks are produced at several small co-ops in different states.
A spherical balloon has a radius of 10 centimeters.
The new radius is \(10+h\text{,}\) so the new volume is \(V(10 + h) = \dfrac{4}{3}\pi(10 + h)^3 \text{ cm}^3\)
We are looking for the difference between \(V(10+2)\) and \(V(10)\text{:}\)
The volume increased by approximately \(3049.44 \text{ cm}^3\)
The beauty of using the \(f(x)\) notation is you can input things that are not numbers. The next example will show you the beauty of this!
Evaluate the function \(f(x)=4x^2 - x + 5\) for the following expressions.
\(\begin{aligned} f(\alert{2h}) \amp= 4(\alert{2h})^2-(\alert{2h}) + 5\\ \amp= 4(4h^2)-2h+5\\ \amp= 16h^2 - 2h + 5\\ \end{aligned}\)
\(\begin{aligned} f(\alert{a+3}) \amp= 4(\alert{a+3})^2-(\alert{a+3})+5\\ \amp= 4(a^2+6a+9)-a-3+5\\ \amp= 4a^2+24a+36 - a + 2\\ \amp= 4a^2+23a + 38\\ \end{aligned}\)
In Example40, notice that
and
To compute \(f(a) + f(3)\text{,}\) we must first compute \(f(a)\) and \(f(3)\text{,}\) then add them:
In general, it is not true that \(f(a + b) = f(a) + f(b)\text{.}\) Remember that the parentheses in the expression \(f(x)\) do not indicate multiplication, so the distributive law does not apply to the expression \(f(a + b)\text{.}\)
Let \(f(x) = x^3 - 1\) and evaluate each expression.
\(f(2) + f(3)=(2^3-1)+(3^3-1)=33\)
\(f(2+3)=f(5)=5^3-1=124\)
\(2 f(x)+3=2 (x^3-1)+3=2x^3 + 1\)
In applied problems, the inputs and outputs of a function come with their own units. Working with these units will allow you to make reasonable interpretations for both the inputs and the outputs of a function. Consider the following example.
Let \(f(t)\) be the total number of reported flu cases at UNL by the \(t^{th}\) day of the semester. Then the units of the inputs are days
and the units of the outputs are flu cases
.
What does \(f(103)\) mean?
What does \(f(50)\) mean?
What does \(f(15) = 73\) mean?
What is the meaning of \(x\) in the statement \(f(x) = 56\text{?}\)
What does \(f(103)\) mean?
\(f(103)\) gives the number of flu cases by the 103rd day of the semester.
What does \(f(50)\) mean?
\(f(15)\) gives the number of flu cases by the 50th day of the semester.
What does \(f(15) = 73\) mean?
\(f(15)=73\) means that there were 73 flu cases by the 15th day of the semester.
What is the practical meaning of \(x\) in the statement \(f(x) = 56\text{?}\)
In the equation \(f(x) = 56\text{,}\) \(x\) would give the day(s) by which there were 56 flu cases.
Consider the function \(f\) defined by the following table.
\(x\) | -2 | -1 | 0 | 1 | 2 |
\(f(x)\) | 4 | 5 | 6 | 1 | 2 |
What value of \(x\) will satisfy \(f(x)=6\text{?}\)
By inspecting the table we see that \(f(x)=6\) when \(x=0\text{.}\)