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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:
Linear relationships are relationships in which the rate of change is constant.
A linear function is a function which has a constant rate of change.
The rate of change is the slope of the graph, and the initial value is the value of \(f(0)\text{.}\) On the graph of \(f\text{,}\) the initial value corresponds to the point that lies on the vertical axis.
A vertical intercept is a point where a graph intersects the vertical axis. For a function \(f\text{,}\) the vertical intercept of the graph of \(f\) is the point \((0,f(0))\text{.}\) Because we often use the variable \(y\) on the vertical axis, the vertical intercept is often called the \(y\)-intercept.
We will often abuse notation and say that the \(y\)-intercept of a graph is \(b\text{.}\) What we really mean is that the \(y\)-intercept is the point \((0,b)\text{.}\)
We can write the equation for a linear function explicitly as
where the constant term, \(b\text{,}\) is the \(y\)-intercept and \(m\text{,}\) the coefficient of \(x\text{,}\) is the slope of the line. This form for an equation of a line is called the slope-intercept form.
If we write an equation of a linear function in the form,
then \(m\) is the slope of the line, and \(b\) is the \(y\)-coordinate of the \(y\)-intercept of the line.
(You may have encountered the slope-intercept equation in the equivalent form \(y = mx + b\text{.}\))
For example, consider the two linear functions and their graphs shown in Figure120 and Figure121.
\(f (x) = 10 - 3x\)
\(g(x) = -3+2x\)
Some observations:
By examining the table of values, we can also see why the coefficient of \(x\) gives the slope of the line:
For each graph, the coefficient of \(x\) is a scale factor that tells us how many units \(y\) changes for \(1\) unit increase in \(x\text{.}\) But that is exactly what the slope tells us about a line.
Is is also useful to introduce the term \(x\)-intercept at this point.
An \(x\)-intercept is a point where a graph intersects the \(x\)-axis. It corresponds to the point \((a,0)\) where \(a\) is a value such that \(f(a)=0\text{.}\)
Francine is choosing an Internet service provider. She paid $30 for a modem, and she is considering three companies for service:
Match the graphs in Figure123 to Francine's Internet cost with each company.
Francine pays the same initial amount, $30 for the modem, under each plan. The monthly fee is the rate of change of her total cost, in dollars per month. We can write a formula for her cost under each plan.
The graphs of these three functions all have the same \(y\)-intercept, but their slopes are determined by the monthly fees. The steepest graph, III, is the one with the largest monthly fee, peoplepc, and ISP.com, which has the lowest monthly fee, has the least steep graph, I.
Delbert decides to use DSL for his Internet service.
Write a formula for Delbert's Internet costs under each plan.
Match Delbert's Internet cost under each company with its graph in Figure125.
Earthlink: \(f (x) = 99 + 39.95x\text{;}\) DigitalRain: \(g(x) = 50 + 34.95x\text{;}\) FreeAmerica: \(h(x) = 149 + 34.95x\)
DigitalRain: I; Earthlink: II; FreeAmerica: III
In the equation \(f (x) = b + mx\text{,}\) we call \(m\) and \(b\) parameters. Their values are fixed for any particular linear equation; for example, in the equation \(y = 2x + 3\text{,}\) \(m = 2\) and \(b = 3\text{,}\) and the variables are \(x\) and \(y\text{.}\) By changing the values of \(m\) and \(b\text{,}\) we can write the equation for any line except a vertical line (see Figure127). The collection of all linear functions \(f (x) = b + mx\) is called a two-parameter family of functions.
Look again at the lines in Figure127: There is only one line that has a given slope and passes through a particular point. That is, the values of \(m\) and \(b\) determine the particular line. The value of \(b\) gives us a starting point, and the value of \(m\) tells us which direction to go to plot a second point. Thus, we can graph a line given in slope-intercept form without having to make a table of values.
The slope of a line is a ratio and can be written in many equivalent ways. In Example128, the slope is equal to \(\dfrac{8}{6}\text{,}\) \(\dfrac{12}{9}\text{,}\) and \(\dfrac{-4}{-3}\text{.}\) We can use any of these fractions to locate a third point on the line as a check. If we use \(m = \dfrac{\Delta y}{\Delta x}= \dfrac{-4}{-3}\text{,}\) we move down \(4\) units and left \(3\) units from the \(y\)-intercept to find the point \((-3, -6)\) on the line.
Plot the point associated with the vertical intercept, \((0, b)\text{.}\)
Use the definition of slope to find a second point on the line: Starting at the vertical intercept, move \(\Delta y\) units in the \(y\)-direction and \(\Delta x\) units in the \(x\)-direction. Plot a second point at this location.
Use an equivalent form of the slope to find a third point, and draw a line through the points.
Write the equation \(2y + 3x + 4 = 0\) in slope-intercept form.
Use the slope-intercept method to graph the line.
\(y=-2-\dfrac{3}{2}x \)