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### Course: Get ready for AP® Calculus > Unit 2

Lesson 4: Average rate of change# Introduction to average rate of change

What's the average rate of change of a function over an interval?

## Want to join the conversation?

- While finding average of numbers,etc., we usually add up all those and divide by their count,but in here to find the average speed, we are actually taking up the slope formula.Would anyone please explain . Or am I thinking it in a wrong way?(8 votes)
- On a position-time graph, the slope at any particular point is the velocity at that point. This is because velocity is the rate of change of position, or change in position over time. Here, the average velocity is given as the total change in position over the time taken (in a given interval).

Using your idea of an average, to find the average velocity we'd want to measure the velocity at a bunch of (evenly spaced) points in that interval, and find the average of those. The question you might ask then would be: how many points should we take?

If we just took 2 points (the start and the end), we might get some idea of the average but this would likely be a bad representation of the true average. If the car started off stationary and ended stationary, its velocity is zero at those two points, which would suggest it's average velocity was zero - that can't be right! By taking just two points, we lost all the information about what happened between those points.

So we have to take some more points, and the more points we take, the more information we take into account, and so the closer our estimation should get to the actual answer. In fact, it seems like if we were able to take an infinite number of points we'd get the most accurate value possible. But since infinity is hard to do, let's just use a "large" number instead. So now we have two ways of finding an average velocity, Sal's way and your way. So you now may ask, what's the difference, what makes his way right and my way wrong? In fact, there is no difference, the two ways will give exactly the same answer!

An exact proof of this requires calculus or limits, but you could play around with this idea on paper or on a computer or even run some experiments to test this for yourself.(31 votes)

- Hi! I was wondering what the ∆ symbol means and where it can be used. Thank you!(7 votes)
- The symbol is the Greek letter called delta. It is commonly used as a abbreviation for "change in" something.

For example: ∆y means "change in y".

Hope this helps.(24 votes)

- 5:40Why that line is called secant line?(6 votes)
- A secant line is a line that intersects a curve of some sort, at two points. A secant line is what we use to find average rates of change.(10 votes)

- This video has a mistake at the end. The d(x) for 3 is 10, not 9, and that makes the drawing more logical.(8 votes)
- In past videos, Sal showed the slope-intercept form of an equation (y=mx+b). Could we use that to represent a function? f(x) = mx + b, m being the slope of the function?

And if so, in the function f(t) = t²+1, which is the same as saying t*t+1, why couldn't we say that the slope of that function is t?(5 votes)- Yes, you could say m represents the slope in the
*linear*function f(x)=mx+b.

The function f(t)=t^2 + 1 in your example is not linear (the graph isn’t a line). You can’t find the slope of a function that isn’t linear. There is a concept called a derivative that you’ll learn about in calculus, and it is like slope but for curves. The derivative of t^2+1 is 2t.

(Not just t, you’ll learn why in calc). This means that the slope changes depending on the values of t. For example, the slope of the curve at t=2 is 4 but at t=5 it is 10. The “slope” of lines is constant throughout the line, but the “slope” of a curve changes!(6 votes)

- Is it possible to find the rate of change (as a formula, linear or polynomial) for literally every changing thing in the universe? What do you guys think?(5 votes)
- Not sure about 'everything', but differentiation in Calculus is about getting the exact rate of change of a function at any given point!

The average rate of change takes two points and calculates`(y2 - y1)/(x2 - x1)`

. It's not really accurate for the actual rate of change at the first point, though. So by bringing the second point closer and closer to the first point, we get closer and closer to the actual rate of change at that first point. There's more to it, but this is the basic idea that underlies differential calculus.1:50-2:33in the video, Sal shows this visually, where the slope of the tangent line to any point on the graph is the 'instantaneous rate of change'. A tangent line touches the graph locally at only`1`

point, not`2`

, which reflects on this idea of moving the 2nd point closer and closer to the 1st point, to make it closer to a tangent line.

With differentiation, you can start with a function`f`

and then obtain a function which describes the exact rate of change of`f`

at any point on its graph, called the*derivative of*.`f`

It is possible to find the derivative of many types of elementary functions, including polynomials, exponentials, trigonometric functions, and so on... It's all in the Calculus course! https://www.khanacademy.org/math/ap-calculus-ab(3 votes)

- for d(t) = t^2+1, if t is 1, d(t) is 1+1 so its 2, not 1/1, isnt it?(4 votes)
- Sal is not finding d(t) when t=1. He is finding the average rate of change on the graph d(t) using the 2 points (0,1) and (1,2). The average rate of change is finding the slope between those 2 points.

Slope = Change of Y/Change in X = (y2-y1)/(x2-x1). In this case, it becomes Change of D/Change of t. Sal didn't write out the work, but he walks you thru it verbally.(3 votes)

- I don't get this at all! Can anyone help?(5 votes)
- 5:25I don't understand this part, what does he mean by tangent line and secant line?(3 votes)
- A secant line intersects two points on a curve. The slope of a line secant to a curve gives the average rate of change between those points.

A tangent line just barely “kisses” a curve at a single point. The slope of the tangent line represents the instantaneous rate of change of the curve at that point. Don’t worry about this too much right now; you’ll learn more about it if you take a calculus class.(3 votes)

- What relationship does a tangent line in graphs have with the tangent of a circle?How about secant lines?(4 votes)

## Video transcript

- So we have different definitions for d of t on the left and the right and let's say that d is
distance and t is time, so this is giving us our
distance as a function of time, on the left, it's equal to 3t plus one and you can see the graph
of how distance is changing as a function of time here is a line and just as a review from algebra, the rate of change of a line, we refer to as the slope of a
line and we can figure it out, we can figure out, well,
for any change in time, what is our change in distance? And so in this situation, if we're going from time
equal one to time equal two, our change in time,
delta t is equal to one and what is our change in distance? We go from distance is
equal to four meters, at time equals one, to distance in seven
meters at time equal two and so our change in distance
here is equal to three and if we wanna put our units, it's three meters for
every one second in time and so our slope would be
our change in our vertical divided by our change in our horizontal, which would be change in
d, delta d over delta t, which is equal to three over one or we could just write that
as three meters per second and you might recognize this as a rate, if you're thinking about
your change in distance over change in time,
this rate right over here is going to be your speed. This is all a review of
what you've seen before and what's interesting about a line, or if we're talking
about a linear function, is that your rate does
not change at any point, the slope of this line
between any two points is always going to be three, but what's interesting about
this function on the right is that is not true, our rate of change is constantly changing and we're going to study
that in a lot more depth, when we get to differential calculus and really this video's a little bit of a foundational primer
for that future state, where we learn about differential calculus and the thing to appreciate here is think about the instantaneous
rate of change someplace, so let's say right over there, if you ever think about
the slope of a line, that just barely touches this graph, it might look something like that, the slope of a tangent line and then right over here, it looks like it's a little bit steeper and then over here, it looks
like it's a little bit steeper, so it looks like your rate of change is increasing as t increases. As I mentioned, we will build the tools to later think about
instantaneous rate of change, but what we can start to think about is an average rate of change, average rate of change, and the way that we think about
our average rate of change is we use the same tools, that
we first learned in algebra, we think about slopes of secant lines, what is a secant line? Well, we talk about this in geometry, that a secant is something
that intersects a curve in two points, so let's
say that there's a line, that intersects at t equals
zero and t equals one and so let me draw that
line, I'll draw it in orange, so this right over here is a secant line and you could do the
slope of the secant line as the average rate of change from t equals zero to t equals one, well, what is that average
rate of change going to be? Well, the slope of our
secant line is going to be our change in distance
divided by our change in time, which is going to be equal to, well, our change in time is one second, one, I'll put the units here, one second and what is our change in distance? At t equals zero or d of zero is one and d of one is two, so our distance has
increased by one meter, so we've gone one meter in one second or we could say that our
average rate of change over that first second from t equals zero, t equals one is one meter per second, but let's think about what it is, if we're going from t equals two to t equals three. Well, once again, we can
look at this secant line and we can figure out its slope, so the slope here,
which you could also use the average rate of change from t equals two to t equals three, as I already mentioned, the rate of change seems
to be constantly changing, but we can think about
the average rate of change and so that's going to
be our change in distance over our change in time, which is going to be equal
to when t is equal to two, our distance is equal to five, so one, two, three, four, five, so that's five right over there and when t is equal to three,
our distance is equal to 10, six, seven, eight, nine, 10,
so that is 10 right over there, so our change in time, that's
pretty straightforward, we've just gone forward one
second, so that's one second and then our change in
distance right over here, we go from five meters to
10 meters is five meters, so this is equal to five meters per second and so this makes it very clear, that our average rate
of change has changed from t equals zero, t equals one to t equals two to t equals three, our average rate of change is higher on this second interval,
than on this first one and as you can imagine, something very interesting to think about is what if you were to take the slope of the secant line of
closer and closer points? Well, then you would get closer and closer to approximating that
slope of the tangent line and that's actually what we
will do when we get to calculus.