Before we delve into function values, let’s recall the order of operations to evaluate algebraic expressions.

Order of Operations

When we evaluate algebraic expressions we use the order of operations.

Operations are applied from left to right within terms of the expression. For example, to evaluate

[latex]6 \div 2 \times 4[/latex]

we perform the division before the multiplication:

[latex]\begin{aligned}&6 \div 2 \times 4\\&=3\times 4\\&=12\end{aligned}[/latex]

Multiplication or division are applied before addition or subtraction. For example, to evaluate

[latex]3+4\times5-6[/latex]

we evaluate [latex]4\times5[/latex] before adding and subtracting:

[latex]\begin{aligned}&3+4\times5-6\\&=3+20-6\\&=23-6\\&=17\end{aligned}[/latex]

If there are any exponents or radicals, we evaluate them before performing any multiplication or division, before performing any addition or subtraction. There is a hierarchy to the operations. For example, to evaluate

[latex]3+5\times3^2[/latex]

we evaluate [latex]3^2[/latex] first:

[latex]\begin{aligned}&3+5\times3^2\\&=3+5\times9\\&=3+45\\&=48\end{aligned}[/latex]

Likewise, to evaluate

[latex]\sqrt{2^3-3(2)+3^2+5}[/latex]

we work under the radical first, following the order of operations:

[latex]\begin{aligned}&\sqrt{2^3-3(2)+3^2+5}\\&=\sqrt{8-6+9+5}\\&=\sqrt{16}\\&=4\end{aligned}[/latex]

Notice that we evaluated each term under the radical separately, but at the same time. This is the most efficient method. Recall that a term is a number (or variable), or a product or quotient of numbers (and variables). Terms are separated by addition and subtraction.

Grouping symbols include parentheses, brackets, braces, and absolute values, and the operations inside these should be performed before any other operations. If there are multiple grouping symbols, the expression should be evaluated from the inside out. For example, to evaluate

[latex]5\times[2+(3\times3^2+1)]\div10[/latex]

we complete the operations inside the parentheses first:

[latex]\begin{aligned}&5\times[2+(3\times3^2+1)]\div10\\&=5\times[2+(3\times9+1)]\div10\\&=5\times[2+(27+1)]\div10\\&=5\times[2+(28)]\div10\\&=5\times[30]\div10\\&=150\div10\\&=15\end{aligned}[/latex]

We can also evaluate an algebraic expression if we are given the values assigned to each variable.

Assigning the Independent Variable a Value

Determining function values

The algebraic form of a function defines the independent and dependent variables of the function. For example, the function [latex]f(x)=2x+1[/latex], designates [latex]x[/latex] as the the independent variable. We know this because [latex]x[/latex] is the variable in parentheses next to the function name, [latex]f[/latex]. The dependent variable is the function value, and is dependent on the value of the independent variable. The values of the independent variable determine the values of the function (the dependent variable) through the rule described in the algebraic form of the function.

For example, the function[latex]f(x)=2x+1[/latex] has a function value of 1 when the independent variable [latex]x[/latex] is 0. To determine this, we substitute [latex]x=0[/latex] in the function:

[latex]f(x)=2x+1\\f(0) = 2(0) + 1 =1[/latex]

The notation [latex]f(0)[/latex] means the function value (or the value of the dependent variable) given the value of the independent variable [latex]x[/latex] is 0. The function value is 1, after the independent variable in the rule is replaced with 0. Therefore, we may write [latex]f(0)=1[/latex]. Similarly, the function value is 11 after the independent variable in the rule is replaced with 5.

[latex]f(x)=2x+1\\f(5) = 2(5) + 1 = 10 + 1 = 11[/latex]

On the coordinate plane, a function is a graph. The graph is composed of those points [latex](x,y)[/latex] where [latex]x[/latex] is the independent variable and [latex]y=f(x)[/latex] is the dependent variable. Therefore, when assigning the independent variable a value (e.g., [latex]x=5[/latex]), it refers to the point on the graph where the [latex]x[/latex]-coordinate is 5. According to the graph in figure 1, we can determine that the [latex]y[/latex]-coordinate is 11 when the [latex]x[/latex]-coordinate is 5 because the point (5, 11) lies on the graph of the function. The point (5, 11) is the point where the vertical line [latex]x=5[/latex] intersects the graph [latex]y=f(x)[/latex].

Why do you think there is no function value at [latex]x=3[/latex] for the function [latex]f(x)=\frac{2x}{x-3}[/latex]? If we look at the graph, we may gain a better understanding. Figure 2 shows the graph of the rational function [latex]f(x)=\frac{2x}{x-3}[/latex]. The reason that [latex]f(3)[/latex] is undefined is because there is a vertical asymptote at [latex]x=3[/latex]. A graph can never cross a vertical asymptote, so there is no function value at [latex]x=3[/latex].

Assigning the Function a Value

Determining the value of the independent variable

Graphical Interpretation

On the coordinate plane, when a function is assigned a value, it refers to the point on the graph where the [latex]y[/latex]-coordinate of the point is the assigned value. For example, [latex]f(x)=7[/latex], refers to a point on the graph where the [latex]y[/latex]-coordinate is 7. If we have the graph of the function we can find the corresponding [latex]x[/latex]-value of the function by determining the [latex]x[/latex]-coordinate of the point where [latex]y=7[/latex]. The graph in figure 3 shows that the point (3, 7) lies on the graph of the function. Therefore, when [latex]y=7,\; x=3[/latex]. This means that the function value of 7 occurs when the independent variable value is 3. The point (3, 7) is the point where the horizontal line [latex]y=7[/latex] intersects the graph [latex]y=f(x)[/latex].

Algebraic Interpretation

Figure 3 showed the graphical interpretation of solving [latex]f(x)=7[/latex] for [latex]f(x)=2x+1[/latex]. We can also determine the [latex]x[/latex]-value algebraically, by setting [latex]f(x)=7[/latex]:

[latex]\begin{aligned}f(x)&= 2x + 1 \\7&=2x+1\end{aligned}[/latex]

The equation [latex]2x+1=7[/latex] is a linear equation in one variable. In this case the one variable is [latex]x[/latex], and the equation is linear because the highest power on the variable, [latex]x[/latex], is 1 (i.e., [latex]x=x^1[/latex]). Linear equations in one variable can be written in the form [latex]ax+b=c[/latex], where [latex]a, b[/latex], and [latex]c[/latex] are real numbers with [latex]a\ne 0[/latex].

To solve this linear equation (find the value of [latex]x[/latex] that makes the equation true), we must recall the properties of equality:

Consequently, to solve [latex]7=2x+1[/latex], we start by isolating the [latex]x[/latex]-term by subtracting 1 from both sides of the equation:

[latex]\begin{aligned}7&=2x+1\\7\color{blue}{-1}&=2x+1\color{blue}{-1}\\6&=2x\end{aligned}[/latex]

Now we can isolate the variable, [latex]x[/latex], by dividing both sides of the equation by 2:

[latex]\begin{aligned}6&=2x\\6\color{blue}{÷2}&=2x\color{blue}{÷2}\\3&=x\end{aligned}[/latex]

Therefore, using the reflection property, the solution of the equation [latex]7=2x+1[/latex] is [latex]x=3[/latex].

Notice that this is exactly the same solution that we found graphically in figure 3.

This equation solving process to determine the value of an independent variable given a function value will be discussed throughout this entire course.

The goal of solving any equation is to isolate the variable so that the variable (e.g., [latex]x[/latex]) is on one side and a number is on the other side of the equation (e.g., [latex]x=3[/latex]). A general strategy for solving a linear equation is to first simplify the equation by combining like terms on each side of the equation, then use the addition property of equality to get all the variable terms on one side of the equation and all the constant terms (numbers) on the other side of the equation. Finally, after simplifying by combining like terms if necessary, use the multiplication property of equality to isolate the variable.

For example, the equation [latex]3x + 5 - 10 = 4x - 7 + 8x[/latex] can be solved by first combining the constant terms 5 and –10 on the left side and the two variable terms [latex]4x[/latex] and [latex]8x[/latex] on the right side of the equation:

[latex]\begin{aligned}3x+5-10&=4x-7+8x\\3x + \color{blue}{(5 - 10)} &= \color{blue}{(4x +8x)} - 7 \\3x - 5 &= 12x - 7\end{aligned}[/latex]

Now we want all the [latex]x[/latex]-terms on one side and the constant terms on the other side of the equation. To get all the [latex]x[/latex]-terms on the left side of the equation, we subtract [latex]12x[/latex] from both sides of the equation. To get all the numbers on the right side of the equation, we add 5 to both sides:

[latex]\begin{equation}\begin{aligned}3x - 5 &= 12x - 7 \\ 3x - 5 \color{blue}{- 12x} &= 12x - 7\color{blue}{- 12x} \\-9x - 5 &= - 7 \\-9x -5 \color{blue}{+ 5} &= -7\color{blue}{+ 5}\\-9x &= -2\end{aligned}\end{equation}[/latex]

Finally, we can isolate the variable [latex]x[/latex]. Since it is multiplied by [latex]-9[/latex], we divide both sides of the equation by [latex]-9[/latex]:

[latex]\begin{aligned}-9x &= -2\\-9x\color{blue}{\div -9}&=-2\color{blue}{\div -9}\\x&=\dfrac{-2}{-9}\\x&=\dfrac{2}{9}\end{aligned}[/latex].

Therefore, the solution of the equation [latex]3x + 5 - 10 = 4x - 7 + 8x[/latex] is [latex]x=\dfrac{2}{9}[/latex].