Properties of Functions and Basic Function Types

As we have seen in some examples above, we can represent a function using a graph. Graphs display a great many input-output pairs in a small space. The visual information they provide often makes relationships easier to understand. By convention, graphs are typically constructed with the input values along the horizontal axis and the output values along the vertical axis.

The most common graphs name the input value $x$ and the output value $y$, and we say $y$ is a function of $x$, or $y=f(x)$ when the function is named $f$. The graph of the function is the set of all points $(x, y)$ in the plane that satisfies the equation $y=f(x)$. If the function is defined for only a few input values, then the graph of the function is only a few points, where the $x$-coordinate of each point is an input value and the $y$-coordinate of each point is the corresponding output value. For example, the black dots on the graph in Figure 7 tell us that $f(0)=2$ and $f(6)=1$. However, the set of all points $(x, y)$ satisfying $y=f(x)$ is a curve. The curve shown includes $(0,2)$ and $(6,1)$ because the curve passes through those points.

Figure 7

The vertical line test can be used to determine whether a graph represents a function. If we can draw any vertical line that intersects a graph more than once, then the graph does not define a function because a function has only one output value for each input value. See Figure 8.

Figure 8

HOW TO

Given a graph, use the vertical line test to determine if the graph represents a function.

1. Inspect the graph to see if any vertical line drawn would intersect the curve more than once.

2. If there is any such line, determine that the graph does not represent a function.

EXAMPLE 14

Applying the Vertical Line Test

Which of the graphs in Figure 9 represent(s) a function $y=f(x) ?$

Figure 9

Solution

If any vertical line intersects a graph more than once, the relation represented by the graph is not a function. Notice that any vertical line would pass through only one point of the two graphs shown in parts (a) and (b) of Eigure 9. From this we can conclude that these two graphs represent functions. The third graph does not represent a function because, at most $x$-values, a vertical line would intersect the graph at more than one point, as shown in Figure 10.

Figure 10

TRY IT #11

Does the graph in Figure 11 represent a function?

Figure 11

Source: Rice University, https://openstax.org/books/college-algebra/pages/3-1-functions-and-function-notation
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