How to evaluate functions from a table

When we have a function in formula form, it is usually a simple matter to evaluate the function. For example, the function [latex]f\left(x\right)=5 - 3^[/latex] can be evaluated by squaring the input value, multiplying by 3, and then subtracting the product from 5.

How To: EVALUATE A FUNCTION Given ITS FORMula.

Replace the input variable in the formula with the value provided.
Calculate the result.

Example: Evaluating Functions

Given the function [latex]h\left(p\right)=

^+2p[/latex], evaluate [latex]h\left(4\right)[/latex].

Show Solution

To evaluate [latex]h\left(4\right)[/latex], we substitute the value 4 for the input variable [latex]p[/latex] in the given function.

Therefore, for an input of 4, we have an output of 24 or [latex]h(4)=24[/latex].

In the following video we offer more examples of evaluating a function for specific x values.

Example: Evaluating Functions at Specific Values

For the function, [latex]f\left(x\right)=^+3x - 4[/latex], evaluate each of the following.

[latex]f\left(2\right)[/latex]
[latex]f(a)[/latex]
[latex]f(a+h)[/latex]
[latex]\dfrac[/latex]

Show Solution

Replace the [latex]x[/latex] in the function with each specified value.

[latex]\beginf\left(2\right)&=^+3\left(2\right)-4 \\ &=4+6 - 4 \\ &=6\hfill \end[/latex] [latex]f\left(a\right)=^+3a - 4[/latex] [latex]\beginf\left(a+h\right)&=<\left(a+h\right)>^+3\left(a+h\right)-4 \\[2mm] &=^+2ah+^+3a+3h - 4 \end[/latex] [latex]f\left(a+h\right)=^+2ah+^+3a+3h - 4[/latex] and we know that [latex]f\left(a\right)=^+3a - 4[/latex]

Try It

Given the function [latex]g\left(m\right)=\sqrt[/latex], evaluate [latex]g\left(5\right)[/latex].

Show Solution

In the next video, we provide another example of how to solve for a function value.

Example: Solving Functions

Given the function [latex]h\left(p\right)=

^+2p[/latex], solve for [latex]h\left(p\right)=3[/latex].

Show Solution

If [latex]\left(p+3\right)\left(p - 1\right)=0[/latex], either [latex]\left(p+3\right)=0[/latex] or [latex]\left(p - 1\right)=0[/latex] (or both of them equal 0). We will set each factor equal to 0 and solve for [latex]p[/latex] in each case.

[latex]\begin&p+3=0, &&p=-3 \\ &p - 1=0, &&p=1\hfill \end[/latex]

This gives us two solutions. The output [latex]h\left(p\right)=3[/latex] when the input is either [latex]p=1[/latex] or [latex]p=-3[/latex].

Graph of a <a href= parabola with labeled points (-3, 3), (1, 3), and (4, 24)." width="487" height="459" />

We can also verify by graphing as in Figure 5. The graph verifies that [latex]h\left(1\right)=h\left(-3\right)=3[/latex] and [latex]h\left(4\right)=24[/latex].

Try It

Given the function [latex]g\left(m\right)=\sqrt[/latex], solve [latex]g\left(m\right)=2[/latex].

Show Solution

Evaluating Functions Expressed in Formulas

Some functions are defined by mathematical rules or procedures expressed in equation form. If it is possible to express the function output with a formula involving the input quantity, then we can define a function in algebraic form. For example, the equation [latex]2n+6p=12[/latex] expresses a functional relationship between [latex]n[/latex] and [latex]p[/latex]. We can rewrite it to decide if [latex]p[/latex] is a function of [latex]n[/latex].

How To: Given a function in equation form, write its algebraic formula.

Solve the equation to isolate the output variable on one side of the equal sign, with the other side as an expression that involves only the input variable.
Use all the usual algebraic methods for solving equations, such as adding or subtracting the same quantity to or from both sides, or multiplying or dividing both sides of the equation by the same quantity.

Example: Finding an Equation of a Function

Express the relationship [latex]2n+6p=12[/latex] as a function [latex]p=f\left(n\right)[/latex], if possible.

Show Solution

To express the relationship in this form, we need to be able to write the relationship where [latex]p[/latex] is a function of [latex]n[/latex], which means writing it as [latex]p=[/latex] expression involving [latex]n[/latex].

Therefore, [latex]p[/latex] as a function of [latex]n[/latex] is written as

Analysis of the Solution

It is important to note that not every relationship expressed by an equation can also be expressed as a function with a formula.

Watch this video to see another example of how to express an equation as a function.

Example: Expressing the Equation of a Circle as a Function

Does the equation [latex]^+^=1[/latex] represent a function with [latex]x[/latex] as input and [latex]y[/latex] as output? If so, express the relationship as a function [latex]y=f\left(x\right)[/latex].

Show Solution

First we subtract [latex]^[/latex] from both sides.

We now try to solve for [latex]y[/latex] in this equation.

We get two outputs corresponding to the same input, so this relationship cannot be represented as a single function [latex]y=f\left(x\right)[/latex].

Try It

If [latex]x - 8^=0[/latex], express [latex]y[/latex] as a function of [latex]x[/latex].

Show Solution [latex]y=f\left(x\right)=\cfrac>[/latex]Q & A

Are there relationships expressed by an equation that do represent a function but which still cannot be represented by an algebraic formula?

Yes, this can happen. For example, given the equation [latex]x=y+^[/latex], if we want to express [latex]y[/latex] as a function of [latex]x[/latex], there is no simple algebraic formula involving only [latex]x[/latex] that equals [latex]y[/latex]. However, each [latex]x[/latex] does determine a unique value for [latex]y[/latex], and there are mathematical procedures by which [latex]y[/latex] can be found to any desired accuracy. In this case, we say that the equation gives an implicit (implied) rule for [latex]y[/latex] as a function of [latex]x[/latex], even though the formula cannot be written explicitly.

Evaluating a Function Given in Tabular Form

As we saw above, we can represent functions in tables. Conversely, we can use information in tables to write functions, and we can evaluate functions using the tables. For example, how well do our pets recall the fond memories we share with them? There is an urban legend that a goldfish has a memory of 3 seconds, but this is just a myth. Goldfish can remember up to 3 months, while the beta fish has a memory of up to 5 months. And while a puppy’s memory span is no longer than 30 seconds, the adult dog can remember for 5 minutes. This is meager compared to a cat, whose memory span lasts for 16 hours.

The function that relates the type of pet to the duration of its memory span is more easily visualized with the use of a table. See the table below.

Pet	Memory span in hours
Puppy	0.008
Adult dog	0.083
Cat	16
Goldfish	2160
Beta fish	3600

At times, evaluating a function in table form may be more useful than using equations. Here let us call the function [latex]P[/latex].

The domain of the function is the type of pet and the range is a real number representing the number of hours the pet’s memory span lasts. We can evaluate the function [latex]P[/latex] at the input value of “goldfish.” We would write [latex]P\left(\text\right)=2160[/latex]. Notice that, to evaluate the function in table form, we identify the input value and the corresponding output value from the pertinent row of the table. The tabular form for function [latex]P[/latex] seems ideally suited to this function, more so than writing it in paragraph or function form.

How To: Given a function represented by a table, identify specific output and input values.

Find the given input in the row (or column) of input values.
Identify the corresponding output value paired with that input value.
Find the given output values in the row (or column) of output values, noting every time that output value appears.
Identify the input value(s) corresponding to the given output value.

Example: Evaluating and Solving a Tabular Function

Using the table below,

Evaluate [latex]g\left(3\right)[/latex].
Solve [latex]g\left(n\right)=6[/latex].

[latex]n[/latex]	1	2	3	4	5
[latex]g(n)[/latex]	8	6	7	6	8

Show Solution

Evaluating [latex]g\left(3\right)[/latex] means determining the output value of the function [latex]g[/latex] for the input value of [latex]n=3[/latex]. The table output value corresponding to [latex]n=3[/latex] is 7, so [latex]g\left(3\right)=7[/latex].
Solving [latex]g\left(n\right)=6[/latex] means identifying the input values, [latex]n[/latex], that produce an output value of 6. The table below shows two solutions: [latex]n=2[/latex] and [latex]n=4[/latex].

[latex]n[/latex]	1	2	3	4	5
[latex]g(n)[/latex]	8	6	7	6	8

When we input 2 into the function [latex]g[/latex], our output is 6. When we input 4 into the function [latex]g[/latex], our output is also 6.

Try It

Using the table from the previous example, evaluate [latex]g\left(1\right)[/latex] .

Show Solution [latex]g\left(1\right)=8[/latex]

Finding Function Values from a Graph

Evaluating a function using a graph also requires finding the corresponding output value for a given input value, only in this case, we find the output value by looking at the graph. Solving a function equation using a graph requires finding all instances of the given output value on the graph and observing the corresponding input value(s).

Example: Reading Function Values from a Graph

Given the graph below,

Evaluate [latex]f\left(2\right)[/latex].
Solve [latex]f\left(x\right)=4[/latex].

Graph of a positive parabola centered at (1, 0).

Show Solution

To evaluate [latex]f\left(2\right)[/latex], locate the point on the curve where [latex]x=2[/latex], then read the [latex]y[/latex]-coordinate of that point. The point has coordinates [latex]\left(2,1\right)[/latex], so [latex]f\left(2\right)=1[/latex].
To solve [latex]f\left(x\right)=4[/latex], we find the output value [latex]4[/latex] on the vertical axis. Moving horizontally along the line [latex]y=4[/latex], we locate two points of the curve with output value [latex]4:[/latex] [latex]\left(-1,4\right)[/latex] and [latex]\left(3,4\right)[/latex]. These points represent the two solutions to [latex]f\left(x\right)=4:[/latex] [latex]x=-1[/latex] or [latex]x=3[/latex]. This means [latex]f\left(-1\right)=4[/latex] and [latex]f\left(3\right)=4[/latex], or when the input is [latex]-1[/latex] or [latex]\text[/latex] the output is [latex]\text\text<.>[/latex] See the graph below.

Try It

Using the graph, solve [latex]f\left(x\right)=1[/latex].