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Lesson 16 of 24
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Derivatives as Rates of Change

In this section we look at some applications of the derivative by focusing on the interpretation of the derivative as the rate of change of a function. These applications include acceleration and velocity in physics, population growth rates in biology, and marginal functions in economics.

Amount of Change Formula

One application for derivatives is to estimate an unknown value of a function at a point by using a known value of a function at some given point together with its rate of change at the given point. If  is a function defined on an interval , then the amount of change of  over the interval is the change in the  values of the function over that interval and is given by.

The average rate of change of the function  over that same interval is the ratio of the amount of change over that interval to the corresponding change in the  values. It is given by.

As we already know, the instantaneous rate of change of  at  is its derivative.

For small enough values of . We can then solve for  to get the amount of change formula:.

We can use this formula if we know only  and  and wish to estimate the value of . For example, we may use the current population of a city and the rate at which it is growing to estimate its population in the near future. As we can see in (Figure), we are approximating  by the  coordinate at  on the line tangent to  at . Observe that the accuracy of this estimate depends on the value of  as well as the value of .

Estimating the Value of a Function

If  and , estimate .

Solution

Begin by finding . We have . Thus,.

Given  and , estimate .

-4.4

Hint

Use the same process as in the preceding example.

Motion along a Line

Another use for the derivative is to analyze motion along a line. We have described velocity as the rate of change of position. If we take the derivative of the velocity, we can find the acceleration, or the rate of change of velocity. It is also important to introduce the idea of speed, which is the magnitude of velocity. Thus, we can state the following mathematical definitions.

Definition

Let  be a function giving the position of an object at time .

The velocity of the object at time  is given by .

The speed of the object at time  is given by .

The acceleration of the object at  is given by .

Comparing Instantaneous Velocity and Average Velocity

A ball is dropped from a height of 64 feet. Its height above ground (in feet)  seconds later is given by .

1. What is the instantaneous velocity of the ball when it hits the ground?
2. What is the average velocity during its fall?

Solution

The first thing to do is determine how long it takes the ball to reach the ground. To do this, set . Solving , we get , so it takes 2 seconds for the ball to reach the ground.

1. The instantaneous velocity of the ball as it strikes the ground is . Since  m we obtain  ft/s.
2. The average velocity of the ball during its fall is ft/s.

Interpreting the Relationship between  and

A particle moves along a coordinate axis in the positive direction to the right. Its position at time  is given by . Find  and  and use these values to answer the following questions.

1. Is the particle moving from left to right or from right to left at time ?
2. Is the particle speeding up or slowing down at time ?

Solution

Begin by finding  and .

and .

Evaluating these functions at , we obtain  and .

1. Because , the particle is moving from right to left.
2. Because  and  to decrease. The particle is slowing down.

Position and Velocity

The position of a particle moving along a coordinate axis is given by .

1. Find .
2. At what time(s) is the particle at rest?
3. On what time intervals is the particle moving from left to right? From right to left?
4. Use the information obtained to sketch the path of the particle along a coordinate axis.

Solution

1. The velocity is the derivative of the position function:.
2. The particle is at rest when , so set . Factoring the left-hand side of the equation produces . Solving, we find that the particle is at rest at  and .
3. The particle is moving from left to right when  and from right to left when (Figure) gives the analysis of the sign of  for , but it does not represent the axis along which the particle is moving.The sign of v(t) determines the direction of the particle.Since  on , the particle is moving from left to right on these intervals.
Since  on , the particle is moving from right to left on this interval.
4. Before we can sketch the graph of the particle, we need to know its position at the time it starts moving  and at the times that it changes direction . We have , and . This means that the particle begins on the coordinate axis at 4 and changes direction at 0 and 20 on the coordinate axis. The path of the particle is shown on a coordinate axis in (Figure).The path of the particle can be determined by analyzing .

A particle moves along a coordinate axis. Its position at time  is given by . Is the particle moving from right to left or from left to right at time ?

left to right

Hint

Find  and look at the sign.

Population Change

In addition to analyzing velocity, speed, acceleration, and position, we can use derivatives to analyze various types of populations, including those as diverse as bacteria colonies and cities. We can use a current population, together with a growth rate, to estimate the size of a population in the future. The population growth rate is the rate of change of a population and consequently can be represented by the derivative of the size of the population.

Definition

If  is the number of entities present in a population, then the population growth rate of  is defined to be .

Estimating a Population

The population of a city is tripling every 5 years. If its current population is 10,000, what will be its approximate population 2 years from now?

Solution

Let  be the population (in thousands)  years from now. Thus, we know that  and based on the information, we anticipate . Now estimate , the current growth rate, using.

By applying (Figure) to , we can estimate the population 2 years from now by writing;

thus, in 2 years the population will be approximately 18,000.

The current population of a mosquito colony is known to be 3,000; that is, . If , estimate the size of the population in 3 days, where  is measured in days.

3,300

Use .

Changes in Cost and Revenue

In addition to analyzing motion along a line and population growth, derivatives are useful in analyzing changes in cost, revenue, and profit. The concept of a marginal function is common in the fields of business and economics and implies the use of derivatives. The marginal cost is the derivative of the cost function. The marginal revenue is the derivative of the revenue function. The marginal profit is the derivative of the profit function, which is based on the cost function and the revenue function.

Definition

If  is the cost of producing  items, then the marginal cost  is .

If  is the revenue obtained from selling  items, then the marginal revenue  is .

If  is the profit obtained from selling  items, then the marginal profit  is defined to be .

We can roughly approximate

by choosing an appropriate value for . Since  represents objects, a reasonable and small value for  is 1. Thus, by substituting , we get the approximation . Consequently,  for a given value of  can be thought of as the change in cost associated with producing one additional item. In a similar way,  approximates the revenue obtained by selling one additional item, and  approximates the profit obtained by producing and selling one additional item.

Applying Marginal Revenue

Assume that the number of barbeque dinners that can be sold, , can be related to the price charged, , by the equation .

In this case, the revenue in dollars obtained by selling  barbeque dinners is given by for .

Use the marginal revenue function to estimate the revenue obtained from selling the 101st barbeque dinner. Compare this to the actual revenue obtained from the sale of this dinner.

Solution

First, find the marginal revenue function: .

Hint

Use  to approximate .

Key Concepts

• Using , it is possible to estimate  given  and .
• The rate of change of position is velocity, and the rate of change of velocity is acceleration. Speed is the absolute value, or magnitude, of velocity.
• The population growth rate and the present population can be used to predict the size of a future population.
• Marginal cost, marginal revenue, and marginal profit functions can be used to predict, respectively, the cost of producing one more item, the revenue obtained by selling one more item, and the profit obtained by producing and selling one more item.