# Rate of Change

## Introduction

The rate of change is essentially about how one thing changes in relation to another. For instance, if you're driving a car, your rate of change in distance traveled depends on how your speed changes over time. It's like comparing how much one thing changes compared to how much another thing changes. This concept is super important in math and science, especially when we're dealing with things like motion or growth. In math, we have formulas to help us calculate rate of change in different situations. For example, if we're looking at how fast a car is going, we might use a different formula than if we're looking at how quickly a plant is growing. But the basic idea is always the same, and that is, comparing changes.

## Understanding Rate of Change Formula

The rate of change formula shows how one thing changes in relation to another. It's like figuring out how fast something changes when something else changes. For example, if you're driving a car, the rate of change of your distance traveled depends on how long you have been driving, considering you are driving at a constant speed. We calculate this rate of change using coordinates, like $$(x_1, y_1)$$ and $$(x_2, y_2)$$. It's like measuring how steep a hill is by looking at how much higher you go for every step you take horizontally. In math, we express this as:

\frac{\text{Vertical Change}}{\text{Horizontal Change}} = \frac{\Delta y}{\Delta x} = \frac{y_2 - y_1}{x_2 - x_1}

For a straight line, we can find the rate of change by looking at its slope, represented by 'm' in the equation $$y = mx + b$$.

If we have a function, we can find its rate of change over an interval (a, b) by comparing the function's values at those points. So if you're tracking temperature over time, you'd look at how much it changes in, say, an hour. That's like finding the rate of change of temperature.

## Rate of Change Formula

In mathematics, the rate of change refers to how one quantity changes with respect to another. The rate of change is most commonly termed as slope or gradient. The slope gives an indication about the steepness of a line, meaning how much the dependent variable changes with respect to changes in the independent variable. There are several formulas to calculate this rate.

Formula 1: Basic formula for finding the rate of change:

\text{Rate of change} = \frac{\text{Change in quantity 1}}{\text{Change in quantity 2}}

Formula 2: Rate of change formula in algebra:

\frac{\Delta y}{\Delta x} = \frac{y_2 - y_1}{x_2 - x_1}

Formula 3: Rate of change of Functions:

\frac{f(b) - f(a)}{b - a}

Formula 4: Rate of change for a linear function:

$$y = mx + b$$

Here, rate of change is represented by the slope of the line i.e. $$m$$.

The rate of change can be positive, negative or zero depending upon how the change in the independent variable (x) affects the dependent variable (y).

• If y increases with an increase in x, the rate of change of y with respect to x is positive. For example: The more numbers of hours an electrician works, the more he/she earns.
• If y decreases with an increase in x, the rate of change of y with respect to x is negative. For example: The more number of items you purchase, the lesser is the amount of money remaining in your pocket money.
• If y remains constant with an increase in x, the rate of change of y with respect to x is zero. For example: Your date of birth remains constant as you grow older.

## Applications of Rate of Change Formula

1. Population Growth: The rate of change formula can be used to analyze the growth rate of populations in various regions over time, helping governments and planners make informed decisions regarding resource allocation and infrastructure development.

2. Stock Market Analysis: Investors use rate of change formulas to evaluate the performance of stocks and other financial assets over time. By calculating the rate of change of stock prices, investors can identify trends and make predictions about future market movements.

3. Speed and Velocity: In physics, the rate of change formula is used to calculate speed and velocity. By measuring the change in position over time, scientists can determine the speed and direction of moving objects, which is crucial in fields such as mechanics and kinematics.

4. Chemical Reactions: Chemists use rate of change formulas to study the rates of chemical reactions. By measuring changes in concentration or other properties of reactants and products over time, chemists can determine reaction rates and understand reaction mechanisms.

5. Temperature Change: Engineers use rate of change formulas to analyze temperature changes in various systems. For example, in HVAC (heating, ventilation, and air conditioning) systems, engineers calculate the rate of temperature change to design efficient heating and cooling systems for buildings.

## Solved Examples

Example 1: Given the linear function $$f(x) = 3x + 2$$, find the rate of change.

Solution:

Since the function is linear, the rate of change is equal to the coefficient of $$x$$. Therefore, the rate of change is \frac{df}{dx} = 3.

Example 2: Determine the rate of change between the coordinates $$(2, 3)$$ and $$(4, 9)$$.

Solution:

We have:

$$(x_1, y_1) = (2 , 3)$$

$$(x_2, y_2) = (4 , 9)$$

Using the rate of change formula:

Rate of change  = \frac{\Delta y}{\Delta x}

= \frac{y_2 - y_1}{x_2 - x_1}

= \frac{9 - 3}{4 - 2}

= \frac{6}{2}

= 3

Example 3: Determine the value of $$x$$ for which the rate of change between the points $$(x, 5)$$ and $$(4, 11)$$ is 2.

Solution:

We have:

$$(x_1, y_1) = (x , 5)$$

$$(x_2, y_2) = (4 , 11)$$

\frac{\Delta y}{\Delta x} = 2

Using the rate of change formula:

Rate of change = \frac{\Delta y}{\Delta x} = \frac{y_2 - y_1}{x_2 - x_1}

2 = \frac{11 - 5}{4 - x}

2 = \frac{6}{4 - x}

$$8 - 2x = 6$$

$$2x = 2$$

$$x = 1$$

So, the value of $$x$$ is $$1$$.

Example 4: Find the rate of change of the linear function $$f(x) = 3x - 1$$ over the interval $$[1, 3]$$.

Solution:

We have: $$f(x) = 3x - 1$$

Find the values of $$f(1)$$ and $$f(3)$$.

$$f(1) = 3(1) - 1 = 3 - 1 = 2$$

$$f(3) = 3(3) - 1 = 9 - 1 = 8$$

Using the rate of change formula:

Rate of change = \frac{f(b) - f(a)}{b - a}

= \frac{f(3) - f(1)}{3 - 1}

= \frac{8 - 2}{2}

= \frac{6}{2}

= 3

Example 5: Determine the value of $$y$$ for which the rate of change between the points $$(9, -4)$$ and $$(12, y)$$ is 5.

Solution:

We have:

$$(x_1, y_1) = (9 , -4)$$

$$(x_2, y_2) = (12 , y)$$

\frac{\Delta y}{\Delta x} = 5

Using the rate of change formula:

Rate of change = \frac{\Delta y}{\Delta x} = \frac{y_2 - y_1}{x_2 - x_1}

5 = \frac{y - (-4)}{12 - 9}

5 = \frac{y + 4}{3}

15 = y + 4

y = 11

So, the value of $$y$$ is $$11$$.

Example 6: Josh biked a distance of $$9$$ miles in 1.5 hours at a constant speed. He then took a break for 45 minutes and continued biking for another hour at the same speed. What distance did he bike after the break?

Solution:

Given:

Distance biked before the break: 9 miles

Amount of time Josh biked before break: 1.5 hours

Rate of change of distance over time (constant speed) \frac{9 \text{ miles}}{1.5 \text{ hours}} = 6 miles per hour

Amount of time Josh biked after break: 1 hour

Hence, the distance he biked after the break 1 \text{ hour} \times \frac{6 \text{ miles}}{\text{hour}} = 6 \text{ miles}

Example 7: The graph below displays a function, and it indicates the coordinates of two points on the curve. Use the coordinates to estimate the rate of change of the function between these two points.

Solution:

Let $$(x_1, y_1)$$ be the coordinates of the first point and $$(x_2, y_2)$$ be the coordinates of the second point. The formula for rate of change is:

\frac{y_2 - y_1}{x_2 - x_1}

Let, $$(3, 1)$$ be the first point and $$(5, 8)$$ be the second point. Substituting these values in the above formula, we get:

\text{Rate of change} = \frac{8 - 1}{5 - 3}

= \frac{7}{2}

## Practice Problems

Q1. Given the linear function $$f(x) = 5x + 8$$, find the rate of change.

1. $$8$$
2. $$5$$
3. $$2$$
4. $$4$$

Q2. Find the rate of change of the quadratic function $$f(x) = - 2x + 3$$ over the interval $$[2, 3]$$.

1. $$2$$
2. $$-2$$
3. $$3$$
4. $$-3$$

Q3. Determine the rate of change between the coordinates $$(5, 11)$$ and $$(9, 27)$$.

1. $$4$$
2. $$5$$
3. $$6$$
4. $$7$$

Q4. Determine the value of $$x$$ for which the rate of change between the points $$(x, 3)$$ and $$(1, 6)$$ is 7.

1. \frac{7}{4}
2. \frac{4}{7}
3. \frac{2}{3}
4. \frac{3}{5}

Q5. A hiker is at an altitude of $$400\,\text{feet}$$ after $$2\,\text{hours}$$ and he is at an altitude of $$700\,\text{feet}$$ after $$6\,\text{hours}$$. What is the average rate of change?

1. $$137.5\,\text{feet per hour}$$
2. $$37.5\,\text{feet per hour}$$
3. $$275\,\text{feet per hour}$$
4. $$75\,\text{feet per hour}$$

Q1. What is the rate of change?

Answer: The rate of change represents how one quantity changes with the changes in the other quantity. It is typically calculated as the ratio of the change in one quantity to the change in another. Mathematically, it can be expressed as \frac{\Delta y}{\Delta x}, where $$\Delta y$$ is the change in the dependent variable and $$\Delta x$$ is the change in the independent variable.

Q2. How do you calculate the rate of change for a linear function?

Answer:  For a linear function $$y = mx + b$$, where $$m$$ is the slope and $$b$$ is the y-intercept. The rate of change is simply the slope $$m$$. So, the rate of change is equal to the coefficient of the independent variable $$x$$.

Q3. What is the significance of the rate of change in real-life applications?

Answer: The rate of change is crucial in various real-life scenarios such as determining growth rates, analyzing trends, predicting future outcomes, and understanding the behavior of systems over time. It helps in making informed decisions in fields like economics, science, engineering, and finance.

Q4. How do you interpret negative and positive rates of change?

Answer: A positive rate of change indicates an increase in the dependent variable as the independent variable increases, while a negative rate of change indicates a decrease. This interpretation is essential for understanding trends and behaviors in different contexts.

Q5.  In what situations would you use the average rate of change versus the instantaneous rate of change?

Answer: The average rate of change is useful for analyzing overall trends over a given interval, while the instantaneous rate of change is more precise and is used to understand behavior at a specific point. In real-life applications, the choice between the two depends on the level of detail needed for analysis.