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Derivatives · Derivative

Average rate of change of a function

Before you start, make sure you know: Rate of change, average rate of change, and growth rate (High School Year 1 (semesters 1 and 2))
Before you start, make sure you know: Linear and affine functions (Secondary Year 3)

A bus from Mexico City to Puebla travels 130 km in 2 hours: its average speed is 65 km/h. But that number doesn't tell you whether the driver braked at the toll booth, got stuck in traffic, or sped up on the highway. The same thing happens with any function: the average rate of change measures how much it changes, on average, between two points. Here you will calculate and interpret it, and get it ready for the next step: shrinking the interval until you are left with a single point.

Average rate of change: definition and calculation

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Calculate the average rate of change of the function $f(x) = 3x - 5$ on the interval $[1, 4]$. Does this value match the gradient of the line representing the function?

Calculate the average rate of change of the function $f(x) = 3x - 5$ on the interval $[1, 4]$. Does this value match the gradient of the line representing the function?

Solve
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Find the average rate of change of the quadratic function $g(x) = x^2 - 4x + 1$ on the intervals $[0, 2]$ and $[2, 4]$.

Find the average rate of change of the quadratic function $g(x) = x^2 - 4x + 1$ on the intervals $[0, 2]$ and $[2, 4]$.

Solve
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Given the function $h(t) = t^3 + 2t$, determine its average rate of change on the interval $[-1, 2]$.

Given the function $h(t) = t^3 + 2t$, determine its average rate of change on the interval $[-1, 2]$.

Solve

Geometric interpretation: the secant line

Graph of a curve cut by the secant line joining points A and B of the curve; its slope is the average rate of change. A(a, f(a)) B(b, f(b)) slope of AB = TVM[a, b]

This works with any type of function: polynomial, rational, with radicals. Only the expression you substitute changes.

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Look at the graph of the function $f(x)$ shown in the figure. Calculate the average rate of change on the interval $[-2, 2]$. What geometrical interpretation does this…

Look at the graph of the function $f(x)$ shown in the figure. Calculate the average rate of change on the interval $[-2, 2]$. What geometrical interpretation does this value have with respect to the secant line drawn?

X Y A B f(x) −2 2
Solve
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Given the function $f(x) = -x^2 + 6x - 5$, find the equation of the secant line passing through the points on the graph with x-coordinates $x = 1$ and $x = 4$. How is…

Given the function $f(x) = -x^2 + 6x - 5$, find the equation of the secant line passing through the points on the graph with x-coordinates $x = 1$ and $x = 4$. How is the gradient of this line related to the average rate of change on that interval?

Solve
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The following graph shows the volume of water $V(t)$, in liters, contained in a tank over $8$ hours. From the highlighted points on the graph, calculate the average rate…

The following graph shows the volume of water $V(t)$, in liters, contained in a tank over $8$ hours. From the highlighted points on the graph, calculate the average rate of change of the volume on the intervals $[0, 4]$ and $[4, 8]$. In which period was the emptying faster on average?

t V(t) 4 8 100 80 20
Solve

The ARC on a generic interval $[a, a+h]$

To study what happens near a point $x=a$ it helps to write the other endpoint as $a+h$, where $h$ is the step size. That way the ARC becomes a function of $h$ and you can see what happens as $h$ gets small.

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Find the average rate of change of the radical function $p(x) = \sqrt{x + 3}$ on the interval $[1, 6]$.

Find the average rate of change of the radical function $p(x) = \sqrt{x + 3}$ on the interval $[1, 6]$.

Solve
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Calculate the average rate of change of the function $f(x) = \frac{1}{x}$ on the general interval $[a, a+h]$, simplifying the resulting expression as much as possible.

Calculate the average rate of change of the function $f(x) = \frac{1}{x}$ on the general interval $[a, a+h]$, simplifying the resulting expression as much as possible.

Solve
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Calculate the average rate of change of the function $f(x) = \sqrt{x+2}$ on the interval $[2, 2+h]$. Then, determine which value this rate of change approaches as $h$…

Calculate the average rate of change of the function $f(x) = \sqrt{x+2}$ on the interval $[2, 2+h]$. Then, determine which value this rate of change approaches as $h$ tends to $0$.

Solve

The ARC as average velocity

When $f$ measures a position (or a height) as a function of time, the ARC is the average velocity. It is exactly what we calculated with the bus to Puebla: $\dfrac{130\text{ km}}{2\text{ h}} = 65\text{ km/h}$.

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The distance in meters traveled by a cyclist as a function of time in seconds is given by the expression $d(t) = 2t^2 + 5t$. What was their average speed between the…

The distance in meters traveled by a cyclist as a function of time in seconds is given by the expression $d(t) = 2t^2 + 5t$. What was their average speed between the times $t = 2$ and $t = 5$?

Solve
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The temperature in degrees Celsius of an industrial oven evolves according to the function $T(t) = 20 + 5t - t^2$, where $t$ is time in minutes. Calculate the average…

The temperature in degrees Celsius of an industrial oven evolves according to the function $T(t) = 20 + 5t - t^2$, where $t$ is time in minutes. Calculate the average rate of change of temperature during the first $3$ minutes.

Solve
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A distress flare is launched vertically upward from the top of a cliff. Its height in meters above sea level after $t$ seconds is given by $h(t) = 80 + 30t - 5t^2$.…

A distress flare is launched vertically upward from the top of a cliff. Its height in meters above sea level after $t$ seconds is given by $h(t) = 80 + 30t - 5t^2$. Determine the average velocity of the flare on the intervals $[1, 3]$ and $[3, 5]$. What does the sign of the results obtained indicate?

Solve

Other quantities: average rate of change

The same idea works for any quantity: temperature, population, profits. The ARC tells you how many units of $y$ the quantity changes per unit of $x$. You must always give the units: they are the units of $y$ divided by the units of $x$ (for example, °C/min or thousands of pesos per thousand spent on advertising).

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The monthly profits of a company, in thousands of pesos, are given by the function $B(x) = -2x^2 + 24x - 10$, where $x$ is the amount spent on advertising in thousands…

The monthly profits of a company, in thousands of pesos, are given by the function $B(x) = -2x^2 + 24x - 10$, where $x$ is the amount spent on advertising in thousands of pesos. Calculate the average rate of change of profit if the advertising investment increases from $3$ to $6$ thousand pesos, and interpret the result.

Solve
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The number of bacteria in a laboratory culture after $t$ hours is modelled by the function $N(t) = 500 \times e^{0.2t}$. Calculate the average rate of change of the…

The number of bacteria in a laboratory culture after $t$ hours is modelled by the function $N(t) = 500 \times e^{0.2t}$. Calculate the average rate of change of the population in the time interval between $t = 0$ and $t = 5$ hours. Express the result in terms of the number $e$.

Solve
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Newton's law of cooling states that the temperature of an object approaches ambient temperature exponentially. An engineer models the temperature of a metal component in…

Newton's law of cooling states that the temperature of an object approaches ambient temperature exponentially. An engineer models the temperature of a metal component in degrees Celsius after $t$ minutes using $T(t) = 25 + 150 \times (0,8)^t$. Calculate the average rate of change on the intervals $[0, 5]$ and $[5, 10]$. What conclusion do you draw about the rate of cooling over time?

Solve

Comparing rates and linear functions

Not all functions grow the same way. Comparing the ARC of two functions on the same interval tells you which one changes faster on average there, although on another interval the comparison may reverse.

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Compare the average rate of change of the functions $f(x) = 2^x$ and $g(x) = x^3$ on the interval $[2, 4]$. Which of the two functions experiences greater average growth…

Compare the average rate of change of the functions $f(x) = 2^x$ and $g(x) = x^3$ on the interval $[2, 4]$. Which of the two functions experiences greater average growth on that interval?

Solve
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Prove analytically that the average rate of change of any linear function $f(x) = mx + n$ on an arbitrary interval $[a, b]$ is always constant and equal to $m$.

Prove analytically that the average rate of change of any linear function $f(x) = mx + n$ on an arbitrary interval $[a, b]$ is always constant and equal to $m$.

Solve
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The force of gravitational attraction between the Earth and a satellite varies as a function of the distance $r$ (in thousands of km) from the center of the Earth…

The force of gravitational attraction between the Earth and a satellite varies as a function of the distance $r$ (in thousands of km) from the center of the Earth according to the expression $F(r) = \frac{4 \times 10^5}{r^2}$. Calculate the average rate of change of the force when the distance increases from $10$ to $20$ thousand km. Interpret the meaning of the sign of the result obtained.

Solve

Problems with parameters

Sometimes you are given the ARC and asked for an unknown piece of the function. You set up the formula with the parameter and solve the resulting equation; if there are two parameters, you get two equations, that is, a system.

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Determine the value of the parameter $k$ so that the average rate of change of the function $f(x) = kx^2 - 3x + 2$ on the interval $[1, 3]$ is equal to $5$.

Determine the value of the parameter $k$ so that the average rate of change of the function $f(x) = kx^2 - 3x + 2$ on the interval $[1, 3]$ is equal to $5$.

Solve
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The average rate of change of the function $f(x) = x^3 - 3x^2 + c$ on the interval $[1, b]$ is equal to $6$. Given that $b > 1$, determine the value of $b$. Does the…

The average rate of change of the function $f(x) = x^3 - 3x^2 + c$ on the interval $[1, b]$ is equal to $6$. Given that $b > 1$, determine the value of $b$. Does the result depend on the value of the constant $c$?

Solve
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A quadratic function of the form $f(x) = ax^2 + bx + 5$ is known to have an average rate of change of $1$ on the interval $[0, 2]$ and $9$ on the interval $[2, 4]$. Set…

A quadratic function of the form $f(x) = ax^2 + bx + 5$ is known to have an average rate of change of $1$ on the interval $[0, 2]$ and $9$ on the interval $[2, 4]$. Set up a system of equations and find the values of the parameters $a$ and $b$.

Solve

Piecewise functions and areas with the secant

For a piecewise function, the ARC only depends on the values at the endpoints of the interval: use at each endpoint the rule that applies to it. Whether the function is continuous at the middle point or not does not change the formula, although it does describe how the graph behaves between the endpoints.

Practice★☆☆Not started
Calculate the average rate of change of the rational function $f(x) = \frac{2x + 1}{x - 1}$ on the interval $[2, 5]$.

Calculate the average rate of change of the rational function $f(x) = \frac{2x + 1}{x - 1}$ on the interval $[2, 5]$.

Solve
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Consider the function $f(x) = \frac{4}{x}$. The secant line to the graph of $f(x)$ passing through the points with x-coordinates $x = 1$ and $x = 4$ forms a right-angled…

Consider the function $f(x) = \frac{4}{x}$. The secant line to the graph of $f(x)$ passing through the points with x-coordinates $x = 1$ and $x = 4$ forms a right-angled triangle with the positive coordinate axes. Calculate the area of this triangle.

X Y A B f(x) 1 4
Solve
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Given the piecewise function: $f(x) = \begin{cases} x^2 - 2x & \text{if } x \le 2 \\ \frac{4}{x} - 2 & \text{if } x > 2 \end{cases}$ Calculate the average rate of change…

Given the piecewise function:

$f(x) = \begin{cases} x^2 - 2x & \text{if } x \le 2 \\ \frac{4}{x} - 2 & \text{if } x > 2 \end{cases}$

Calculate the average rate of change on the interval $[0, 4]$. Check whether the function is continuous at $x=2$ and explain whether this affects the calculation of the average rate of change.

Solve

Back to the beginning: the bus to Puebla had an average velocity of $65$ km/h, which is the ARC of its position on $[0,2]$ hours. It says a lot about the whole trip and almost nothing about each instant. For that you have to shrink the interval.

Summary

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