# 12.17: Exponential Growth and Decay

**Practice**Exponential Growth and Decay

Have you ever been to a condominium? Take a look at this dilemma.

A condominium complex charges $185 per month for the homeowners’ association fee. The rates can rise every year because of inflation but they promise not to raise the rates more than 10% each year. Keep in mind, though, that if they raise the rate by 10% the first year, the second year is now more expensive. If they raise the maximum again, they are increasing the original $185 plus the first year’s adjustment by 10%. Graph the situation for 10 years.

How much could the homeowners’ fee be in ten years? Use the function where is the fee after years.

**
Understanding exponential growth and decay will help you with this dilemma. Pay attention and you will see it again at the end of the Concept.
**

### Guidance

**
Do you know how to tell if a function is an exponential function?
**

If your function can be written in the form , where and are constants, and , then it must be exponential. In quadratic equations, your functions were always to the power. In exponential functions, the exponent is a variable. Their graphs will have a characteristic curve either upward or downward.

**
Exponential Functions
**

**
Not Exponential Functions
**

In some cases with exponential functions, as the
value increased, the
value increased, too.
**
This was a
**
**
direct
**

**relationship known as**

*exponential growth***.**As the value increases, the value grows at a very fast rate! The other graph you saw showed the opposite—as the value increased, the value decreased.

**This relationship is an**

*inverse***relationship known as**

*decay***.**The graphs of these functions are opposites, reflected on the -axis.

We can also analyze growth and decay functions in real – life situations.

A famous story tells about a courtier who presented a Persian king with a beautiful handmade chessboard. The king asked him what he would like in return for his gift and the courtier surprised the king by asking him for one grain of rice on the first square of the chessboard, two grains of rice on the second, four grains on the third, etc. The king agreed and ordered for the rice to be brought. By the square, over a million grains of rice were required and, by the square, over a quadrillion grains of rice were needed. There was simply not enough rice in all the world for the final squares.

This story reminds us of the drastic increases that we can see in exponential functions. Although this story is a fable, there are many instances in the real-world where exponential growth can be seen.

Graph each function and tell whether it will represent exponential growth or decay.

#### Example A

**
Solution: Exponential decay
**

#### Example B

**
Solution: Exponential growth
**

#### Example C

**
Solution: Exponential growth
**

Now let's go back to the dilemma from the beginning of the Concept.

First, make a table of values.

0 | 185 |

1 | 203.5 |

2 | 223.85 |

3 | 246.24 |

4 | 270.86 |

5 | 297.94 |

6 | 327.74 |

7 | 360.51 |

8 | 396.56 |

9 | 436.22 |

10 | 479.84 |

Now we graph the function.

### Vocabulary

- Exponential Functions
- results that expand exponentially. The graph curves upward or downward.

- Exponential Growth Graph
- a direct relationship graph each variable increases.

- Decay Graph
- an indirect relationship graph, one variable increases as the other one decreases.

### Guided Practice

Here is one for you to try on your own.

Does the following function represent an exponential function?

**
Solution
**

No, it does not represent an exponential function because the value is 1.

### Video Review

### Practice

Directions: Graph each function. Then say whether it represents economic growth or decay. There will be two answers for each problem.

### Image Attributions

## Description

## Learning Objectives

Here you'll distinguish between exponential growth and exponential decay.