2.3 Rates of Change and Behavior of Graphs
Since functions represent how an output quantity varies with an input quantity, it is natural to ask about the rate at which the values of the function are changing.
For example, the function below gives the average cost, in dollars, of a gallon of gasoline years after 2000.
| t | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
| C(t) | 1.47 | 1.69 | 1.94 | 2.30 | 2.51 | 2.64 | 3.01 | 2.14 |
If we were interested in how the gas prices had changed between 2002 and 2009, we could computer that the cost per gallon had increased from $1.47 to $2.14, an increase of 0.67. While this is interesting, it might be more useful to look at how much the price changed per year. You are probably noticing that the price didn’t change the same amount each year, so we would be finding the average rate of change over a specified amount of time.
The gas price increased by 0.67 from 2002 to 2009, over 7 years, for an average of dollars per year. On average, the price of gas increased by about 9.6 cents each year.
Rate of Change
A rate of change describes how the output quantity changes in relation to the input quantity. The units on a rate of change are “output units per input units”
Some other examples of rates of change would be quantities like:
- A population of rats increases by 40 rats per week
- A barista earns 9 dollars per hour
- A farmer plants 60,000 onions per acre
- A car can drive 27 miles per gallon
- A population of grey whales decreases by 8 whales per year
- The amount of money in your college account decreases by 4,000 dollars per quarter
Average Rate of Change
The average rate of change between two input values is the total change of the function values (output values) divided by the change in the input values.
Example
Using the cost-of-gas function from earlier, find the average rate of change between 2007 and 2009
From the table, in 2007 the cost of gas was 2.64. In 2009 the cost was 2.14.
The input (years) has changed by 2. The output has changed by 2.14 – 2.64 = -0.50. The average rate of change is then dollars per year.
Notice that in the last example the change of output was negative since the output value of the function had decreased. Correspondingly, the average rate of change is negative.
Try it Now 1
Using the same cost-of-gas function, find the average rate of change between 2003 and 2008
Examples
a. On a road trip, after picking up your friend who lives 10 miles away, you decide to record your distance from home over time. Find your average speed over the first 6 hours.
| t (hours) | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
| D(t) (miles) | 10 | 55 | 90 | 153 | 214 | 240 | 292 | 300 |
Here, your average speed is the average rate of change. You traveled 282 miles in 6 hours for an average speed of miles per hour.
b. Given the function shown here, find
the average rate of change on the interval [0,3].
At , the graph shows
At , the graph shows
The output has changed by 3 while the input has changed by 3, giving an average