Copyright © Cengage Learning. All rights reserved. 2 Differentiation Copyright © Cengage Learning. All rights reserved.
Copyright © Cengage Learning. All rights reserved. 2.8 Related Rates Copyright © Cengage Learning. All rights reserved.
Objectives Examine related variables. Solve related-rate problems.
Related Variables
Related Variables In this section, you will study problems involving variables that are changing with respect to time. If two or more such variables are related to each other, then their rates of change with respect to time are also related. For instance, suppose that x and y are related by the equation y = 2x.
Related Variables If both variables are changing with respect to time, then their rates of change will also be related. In this simple example, you can see that because y always has twice the value of x, it follows that the rate of change of y with respect to time is always twice the rate of change of x with respect to time.
Example 1 – Examining Two Rates That Are Related The variables x and y are differentiable functions of t and are related by the equation y = x2 + 3. When x = 1, dx/dt = 2. Find dy/dt when x = 1. Solution: Use the Chain Rule to differentiate both sides of the equation with respect to t.
Example 1 – Solution cont’d When x = 1 and dx/dt = 2, you have
Solving Related-Rate Problems
Example 2 – Changing Area A pebble is dropped into a calm pool of water, causing ripples in the form of concentric circles, as shown in the photo. The radius r of the outer ripple is increasing at a constant rate of 1 foot per second. When the radius is 4 feet, at what rate is the total area A of the disturbed water changing? Total area increases as the outer radius increases
Example 2 – Solution The variables r and A are related by the equation for the area of a circle, A = r 2. To solve this problem, use the fact that the rate of change of the radius is given by dr/dt. Equation: Given rate: Find:
Example 2 – Solution cont’d Using this model, you can proceed as in Example 1.
Example 2 – Solution When r = 4 and dr/dt = 1, you have cont’d When r = 4 and dr/dt = 1, you have When the radius is 4 feet, the area is changing at a rate of 8 square feet per second.
Solving Related-Rate Problems In Example 2, note that the radius changes at a constant rate (dr/dt = 1 for all t), but the area changes at a nonconstant rate.
Solving Related-Rate Problems The solution shown in Example 2 illustrates the steps for solving a related-rate problem.
Solving Related-Rate Problems In Step 2 of the guidelines, note that you must write an equation that relates the given variables. To help you with this step, reference tables that summarize many common formulas are included in the appendices. For instance, the volume of a sphere of radius r is given by the formula
Solving Related-Rate Problems The table below lists examples of the mathematical models for some common rates of change that can be used in the first step of the solution of a related-rate problem.
Example 4 – Increasing Production A company is increasing the production of a product at the rate of 200 units per week. The weekly demand function is modeled by p = 100 – 0.001x where p is the price per unit and x is the number of units produced in a week. Find the rate of change of the revenue with respect to time when the weekly production is 2000 units. Will the rate of change of the revenue be greater than $20,000 per week?
Example 4 – Solution Because production is increasing at a rate of 200 units per week, you know that at time t the rate of change is dx/dt = 200. So, the problem can be stated as shown. Given rate: Find:
Example 4 – Solution cont’d To find the rate of change of the revenue, you must find an equation that relates the revenue R and the number of units produced x. Equation: By differentiating both sides of the equation with respect to t, you obtain
Example 4 – Solution Using x = 2000 and dx/dt = 200, you have cont’d Using x = 2000 and dx/dt = 200, you have No, the rate of change of the revenue will not be greater than $20,000 per week.