Recall that a rate is a ratio between two measurements with different units.
When we graph these rates, the rate of change can be understood as the slope, steepness or slope of a line. Further, we look at the equations in slope-intercept form (that is, $y=ax+b$y=ax+b, where $a$a is the slope), the larger the absolute value of $a$a, the steeper the slope of the line. Other versions of the slope intercept form of a line are $y=mx+b$y=mx+b or $y=mx+c$y=mx+c .
For example, a line with a slope of of $4$4 is steeper than a line with a slope of $\frac{2}{3}$23. Similarly, a line with a slope of $-2$−2 is steeper than a line with a slope of $1$1, even though one is positive and one is negative.
The rate of change in a graph can be positive or negative.
The lines below have positive rates of change. Notice how as the values on the $x$x-axis increase, the values on the $y$y-axis also increase.
These next graphs have decreasing rates of change. Unlike graphs with a positive slope, as the values on the $x$x-axis increase, the values on the $y$y-axis decrease.
The rate of change of a line is a measure of how steep it is. In mathematics we also call this the slope.
The rate of change is a single value that describes:
Take a look at this line, where the horizontal and vertical steps are highlighted:
We call the horizontal measurement the run and the vertical measurement the rise. For this line, a run of $1$1 means a rise of $2$2, so the line has slope $2$2.
Sometimes it is difficult to measure how far the line goes up or down (how much the $y$y value changes) in $1$1 horizontal unit, especially if the line doesn't line up with the grid points on the $xy$xy-plane. In this case we calculate the slope by using a formula:
$\text{slope }=\frac{\text{rise }}{\text{run }}$slope =rise run
Where you take any two points on the line whose coordinates are known or can be easily found, and look for the rise and run between them.
You can find the rise and run of a line by drawing a right triangle created by any two points on the line. The line itself forms the hypotenuse.
This line has a slope of $\frac{\text{rise }}{\text{run }}=\frac{4}{3}$rise run =43
In this case, the slope is positive, because over the $3$3 unit increase in the $x$x-values, the $y$y-value has increased. If the $y$y-value decreased as the $x$x-value increases, the slope would be negative.
This applet allows you to see the rise and run between two points on a line of your choosing.
Horizontal lines have no rise value. The $\text{rise }=0$rise =0.
So the slope of a horizontal line is $\text{slope }=\frac{\text{rise }}{\text{run }}$slope =rise run $=$=$\frac{0}{\text{run}}$0run$=$=$0$0.
Vertical lines have no run value. The $\text{run }=0$run =0.
So the slope of a vertical line is $\text{slope }=\frac{\text{rise }}{\text{run }}$slope =rise run $=$=$\frac{\text{rise }}{0}$rise 0. Division by $0$0 results in the value being undefined.
Description of rate of change: $\text{slope }=\frac{\text{rise }}{\text{run }}$slope =rise run
Slope of vertical line: undefined
Slope of horizontal line: $0$0
In $y=ax$y=ax, $a$a represents the slope of the line
Consider these scenarios:
In each of these cases, we are interested in how one measurement varies as another one does. That is, there is a dependent variable that varies with respect to an independent variable. However, we’re not interested in the measurements themselves but in how they vary. We call this the rate of change.
Let’s focus on the first scenario listed above. We can’t directly measure speed, but we can measure distance and time. Notice that speed is the amount that distance changes per unit of time. That is, speed is the rate of change of distance with respect to time, so we can use our measurements of distance and time to figure out the speed.
Suppose that the distance ($x$x, in meters) is related to the time ($t$t, in seconds) by the relationship $x=3t$x=3t. Let’s plot this relationship first:
Notice that this is a linear function. Since speed is the rate of change of distance over time, we want to find out how the distance changes over any amount of time. Let’s start by picking two points on the line. We’ll use $\left(1,3\right)$(1,3) and $\left(3,9\right)$(3,9):
First we find the change in the independent and dependent variables:
Change in time | $=$= | $3-1$3−1 m |
$=$= | $2$2 m | |
Change in distance | $=$= | $9-3$9−3 s |
$=$= | $6$6 s |
And then we divide the change in distance by the change of time to get the rate of change:
Rate of change | $=$= | $\frac{6}{2}$62 m/s |
$=$= | $3$3 m/s |
So the speed is $3$3 m/s. We will get the same result no matter which two points we choose. Notice that this is the same as the slope of a linear function. In fact, this is always the case when the function is linear.
The rate of change of a dependent variable with respect to an independent variable is how much the dependent variable changes as the independent variable changes.
In the case of a linear function, the rate of change is the slope.
Note that the independent variable is most often time, but can be anything else.
What kind of slope does the following line have?
Positive
Negative
Undefined
Zero
Gas costs a certain amount per gallon. The table shows the cost of various amounts of gas.
Number of gallons ($x$x) | $0$0 | $10$10 | $20$20 | $30$30 | $40$40 |
---|---|---|---|---|---|
Cost of gas ($y$y) | $0$0 | $12.70$12.70 | $25.40$25.40 | $38.10$38.10 | $50.80$50.80 |
Write an equation linking the number of gallons of gas pumped ($x$x) and the cost of the gas ($y$y).
Enter each line of work as an equation.
How much does gas cost per gallon?
How much would $73$73 gallons of gas cost at this unit price?
In the equation, $y=1.27x$y=1.27x, what does $1.27$1.27 represent?
The number of gallons of gas pumped.
The total cost of gas pumped.
The unit rate of cost of gas per gallon.