This section covers:

**Algebraic Functions Versus Relations****Vertical Line Test****Domain and Range of Relations and Functions****Finding the Domain Algebraically****More Practice**

# Algebraic Functions Versus Relations

When we first talked about the coordinate system, we worked with the graph that shows the relationship between how many hours we worked (the independent variable, or the “** x**”), and how much money we made (the dependent variable, or the “

**”). Any relationship between two variables, where one depends on the other, is called a**

*y***relation**, since it relates two things.

This particular relation is an **algebraic** **function**, since there is only one ** y** for each

**. In other words, since the**

*x***is the “question” and**

*x***is the “answer”, we can only have one answer for each question. So for whatever is the number of hours we work, we only get paid a certain amount for that:**

*y*Again, a function is just a fancy way of saying something depends on something else, and there’s only **one** “** y**”

**for every**“

**”. But the other thing you’ll learn about functions is that they can be written a funny way; a way that looks really complicated, but they are just trying to confuse us – it’s not that bad!**

*x*Instead of our original equation, ** y = 10x**, we can write it like this:

\(f(x)=10x\)

Note that this is **not “f times x”**; it is “

**f of**”. What it means is that

*x***is on the**

*x***right hand sign**of the “=” sign, and you can put different values in for

**on the left hand side to get one and only one value on the**

*x***right hand sign**. So again, “

**” is really “**

*f(x)***”. It’s that simple.**

*y*Here are some examples of plugging in things on the left hand side, and then, to get our answer, we plug what that is for **every x **on the right hand side:

Again, what makes a relation a function is that you can only have one “answer” (the ** y**) for each “question” (the

**).**

*x***So all functions are relations, but not all relations are functions.**

# Vertical Line Test

Notice that when we have a function, we can’t draw a vertical that goes through more than one point. This is called the **vertical line test**, and it’s a useful tool to determine if a graph is a function or not. For example, we can tell the following graph is not a function since we can draw a vertical line and hit more than one point:

Here are some examples of functions. Notice that you **can have** two “questions” (** x**) for the same “answer” (

**):**

*y*Another way to look at a set of points and determine whether or not they are functions is to draw what we call **mapping diagrams**, since we are mapping the ** x** values to the

**values. We order values from smallest to largest and don’t repeat the values on each side and match them up. If we have more than one**

*y***value for one**

*y***value, we don’t have a function. Here are some examples:**

*x*# Domain and Range of Relations and Functions

**Domain** and **Range** of functions (and relations) sound really difficult and scary, but they are not really bad at all. You know how those mathematicians like to use fancy words for easy stuff?

Remember that since “**d**” comes before “**r**”, the **domain **of functions have to do with the “** x**”’s and the

**range**of functions have to do with the “

**”’s. To get the domain, we are just looking for all the**

*y***possible**values of

*x***for that function**(from smallest to largest), and for the range, we are looking for all possible values of

*y***for that function**(again, from smallest to largest).

To help me do this, I like to use my pencil – but it’s backwards compared to what you might think. To find the domain, I put my pencil **vertically** and start at the **left** and see where it first hits a point. Then I push it through all the way to the right to see where it ends hitting points.

For the range, I do the same thing, but with a **horizontal** pencil that’s moving **up**:

Here are more examples, using what we call “**Interval Notation**”. (We saw this in the **Inequalities** Section). This is the most commonly used way to describe domains and ranges, and it always goes from lowest to highest with “**(**“ (**soft brackets**) if the relation or function doesn’t hit the point, and “**[**“ (**hard brackets**) if the relation or function does hit the line. If you have to skip over any numbers, you do so by using the “**U**” sign, which means **union**, or putting things together.

We can also use **Inequality Notation**, where, as we saw before, we use inequality signs to describe the ranges.

**Notice on when we see arrows (in the first two examples below), we have to assume that both the domain and range go on forever in those directions.**

If you don’t see how we got the domain and range above, use the pencil trick, and make sure you start from the left for the domain (with vertical pencil) and from the bottom with the range (with horizontal pencil).

Note again that the last two graphs **are not functions**; they do not meet the **Vertical Line Test** requirements.

# Restricted Domains: Finding the Domain Algebraically

In many cases, the **domain is restricted**.

**A domain is restricted if: it is i****ndicated that way in the problem, **and/or **there is a variable in the denominator and the denominator could be zero** and/or** there is a variable underneath an even radical sign and that radicand could be less than zero.**

It could also be restricted if there are other functions in it, like the **log function**, but we’ll learn about that later in the **Logarithmic Functions** section.

We start out assuming that the **domain of a function is all real numbers**, but then see **if there are any exceptions**, as seen in the table. We will learn more about **rational functions** (shown in the first two examples, where there are variables in the denominator) in the** Rational Functions and Equations**, and **Graphing Rational Functions, including Asymptotes** sections.

We will work on more advanced topics with functions later, in the **Advanced Functions** section.

**Learn these rules, and practice, practice, practice!**

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On to **Scatter Plots, Correlation, and Regression **– you are ready!