6-2 Differential Equations: Growth and Decay

Objective: Use separation of variables to solve a simple differential equation; use exponential functions to model growth and decay.

AP CalculusMs. Battaglia

The strategy is to rewrite the equation so that each variable occurs on only one side of the equation. This strategy is called separation of variables. Ex:

y ’ = 2x/y

Solving a Differential Equation

a. b. c.

Practice Separating Variables

A constant rate of growth applied to a continuously growing base over a period of time

What is exponential growth?

In many applications, the rate of change of a variable y is proportional to the value of y. If y is a function of time t, the proportion can be written as follows.

Growth and Decay Models

Rate of change of y

isproportional

to y.

If y is a differentiable function of t such that y > 0 and y’ = ky for some constant k, then

y = Cekt.C is the initial value of y, and k is the proportionality constant. Exponential growth occurs when k > 0, and exponential decay occurs when k < 0.

The rate of change of y is proportional to y. When t=0, y=2, and when t=2, y=4. What is the value of y when t=3?

Using an Exponential Growth Model

The rate of change of y is proportional to y. When t=0, y=6, and when t=4, y=15. What is the value of y when t=8?

Using an Exponential Growth Model

Suppose that 10 grams of the plutonium isotope 239Pu was released in the Chernobyl nuclear accident. How long will it take for the 10 grams to decay to 1 gram?

Radioactive Decay

Radioactive decay is measured in half-life- the number of years required for half of the atoms in a sample of radioactive material to decay. The rate of decay is proportional to the amount present.

Uranium (238U) 4,470,000,000 years

Plutonium (238U) 24,100 yearsCarbon (14C) 5715 years

Radium (226Ra) 1599 yearsEinsteinium

(254Es)276 days

Nobelium (257No) 25 seconds

Suppose an experimental population of fruit flies increases according to the law of exponential growth. There were 100 flies after the 2nd day of the experiment and 300 flies after the 4th day. Approximately how may flies were in the original population?

Population Growth

Four months after it stops advertising, a manufacturing company notices that its sales have dropped from 100,000 units/month to 80,000 units per month. If the sales follow an exponential pattern of decline, what will they be after another 2 months?

Declining Sales

AB: Read 6.2 Page 420 #1-12, 21, 23, 25-28

BC: Read 6.2 Page 420 #7-14, 21, 25-28, 33, 34, 57, 58, 73, 75-78

Classwork/Homework