Here is an index card that goes with this lesson (side 1a only).
The ideas are discussed more fully below.

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to have an in-a-nutshell summary of important calculus ideas.

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Recall that a function is a rule that takes an input, does something to it, and gives a corresponding output.
Function notation is extremely useful for describing this input/output relationship:
if the function name (the rule) is $\,f\,,$ and the input is $\,x\,,$ then the corresponding output is $\,f(x)\,.$
That is, the notation $\,f(x)\,$ (read as ‘$f$ of $x$’) represents the output from the function $\,f\,$ when the input is $\,x\,.$

As inputs change, you often want to know how the corresponding outputs (function values) change.
That is, as inputs change from $\,a\,$ to $\,b\,,$ how is $\,f(a)\,$ changing to $\,f(b)\,$? Are these outputs increasing? Decreasing? Quickly? Slowly?

Here's an example.
Suppose you have a shower dial that controls the temperature of the water.
How do adjustments of the dial (changes in the input) affect the temperature of the water (changes in the output)?
When you move the dial from (say) position $\,a\,$ to position $\,b\,,$ does the water get hotter or colder?
(Perhaps the dial is broken, and the temperature doesn't change at all!)
Does moving the dial from $a$ to $b$ change the temperature a lot, or only a little?
These are the sorts of questions we want answered, and the concept of average rate of change comes to the rescue.

DEFINITION average rate of change of a function $\,f\,$ from $\,a\,$ to $\,b\,$ The average rate of change of $\,f\,$ from $\,a\,$ to $\,b\,$ is: $$\frac{f(b) - f(a)}{b - a} $$ Observations on the definition: Different names for the same thing: the average rate of change of $\,f\,$ from $\,a\,$ to $\,b\,$ the average rate of change of $\,f\,$ on the interval $[a,b]$ the average rate of change of $\,f\,$ between $a$ and $b$ The denominator, $\,b - a\,,$ gives the change in the inputs. The numerator, $\,f(b)-f(a)\,,$ gives the corresponding change in the outputs. The uppercase Greek letter delta ($\Delta$) denotes a change in a quantity. For example, $\Delta x$ can be read as ‘change in $x$’ or ‘delta $x$’, and denotes a change in $x$. The average rate of change of a function $f$ takes the form $$\frac{\Delta{\text{ output}}}{\Delta{\text{ input}}} = \frac{\Delta f(x)}{\Delta x} $$

• The slope of the line through the points $(a,f(a))$ and $(b,f(b))$ is: $$\frac{\text{rise}}{\text{run}} = \frac{\Delta y}{\Delta x} = \frac{f(b)-f(a)}{b-a}\qquad\qquad\text{This is the average rate of change formula!}$$
  Average rate of change gives the slope of the line through the points at the beginning and end of the interval of interest.
• The average rate of change of $f$ from $a$ to $b$ is equal to the average rate of change of $f$ from $b$ to $a$: $$ \overbrace{\frac{f(b)-f(a)}{b-a}}^{\text{from $a$ to $b$}} \cdot\frac{-1}{-1} = \overbrace{\frac{f(a)-f(b)}{a-b}}^{\text{from $b$ to $a$}} $$ This makes sense, since the slope of the line through two points $P_1$ and $P_2$ is the same as the slope of the line through $P_2$ and $P_1$.
• The units of average rate of change are the units of the output divided by the units of the input.
  That is, the units of average rate of change are the units of $f(x)$ divided by the units of $x$.
  
  For example, if $f(t)$ measures the position of a particle on a number line (in feet) at time $t$ (in seconds),
  then the units of average rate of change are feet per second.
  In this case, the average rate of change is the average velocity of the particle on the interval of interest.

You can explore average rate of change using JSXGraph below: