Learning Objective(s)

·Evaluate expressions containing exponents.

·Evaluate exponential notations with exponents of 0 and 1.

·Write an exponential expression involving negative exponents with positive exponents.


Introduction

A common language is needed in order to communicate mathematical ideas clearly and efficiently. Exponential notation is one example. It was developed to write repeated multiplication more efficiently. For example, growth occurs in living organisms by the division of cells. One type of cell divides 2 times in an hour. So in 12 hours, the cell will divide 2 • 2 • 2 • 2 • 2 • 2 • 2 • 2 • 2 • 2 • 2 • 2 times. This can be written more efficiently as 212.


Exponential Vocabulary

We use exponential notation to write repeated multiplication, such as 10 • 10 • 10 as 103The 10 in 10is called the base. The 3 in 10is called the exponent. The expression 10is called the exponential expression.


base    103   ←exponent

103 is read as "10 to the third power” or "10 cubed.” It means 10 • 10 • 10, or 1,000.

82 is read as "8 to the second power” or "8 squared.” It means 8  8, or 64.

54 is read as "5 to the fourth power.” It means 5  5  5  5, or 625.

b5 is read as " b to the fifth power.” It means  b  b  b  b. Its value will depend on the value of b.

The exponent applies only to the number that it is next to. So in the expression xy4, only the y is affected by the 4. xy4 means x • y • y • y • y.


If the exponential expression is negative, such as 34, it means -(3 •  3  3  3) or 81.


If 3 is to be the base, it must be written as (3)4, which means 3, or 81.


Likewise, (x)4 = (x (x (x (x)  = x4, while x4 = -(x  x  x  x).


You can see that there is quite a difference, so you have to be very careful!


Evaluating Expressions Containing Exponents

Evaluating expressions containing exponents is the same as evaluating any expression. You substitute the value of the variable into the expression and simplify.


You can use PEMDAS to remember the order in which you should evaluate the expression. First, evaluate anything in Parentheses or grouping symbols. Next, look for Exponents, followed by Multiplication and Division (reading from left to right), and lastly, Addition and Subtraction (again, reading from left to right).


So, when you evaluate the expression 5x3 if x = 4, first substitute the value 4 for the variable x. Then evaluate, using order of operations.


Example

Problem

Evaluate.

5x3 if x = 4

5 • 43

Substitute 4 for the variable x.

5(4 • 4 • 4) = 5 • 64

Evaluate 43.

320

Multiply.

Answer

5x320 when x = 4


Notice the difference between the example above and the one below.


Example

Problem

Evaluate.

(5x)3 if x = 4

(5 • 4)3

Substitute 4 for the variable x.

203

Multiply.

20 • 20 • 20 = 8,000

Evaluate 203.

Answer

(5x)3 = 8,000 when x = 4


The addition of parentheses made quite a difference!


Example

Problem

Evaluate.

x3if x = 4

(4)3

Substitute 4 for the variablex.

··4

Evaluate.

··4 = 64

Multiply.

Answer

x364, when x = 4


Evaluate the expression  (2x)4, if x = 3.

 

A) 1,296

B) 1,296

C) 162

D) 162


Exponents of Zero and One

What does it mean when an exponent is 0 or 1? Let's consider 251. Any value raised to the power of 1 is just the value itself. This makes sense, because the exponent of 1 means the base is used as a factor only once. So the base stands alone, and 251 is simply 25.


But what about a value raised to the power of 0? Use what you know about powers of 10 to find out what the power of 0 means. Below is a list of powers of 10 and their equivalent values. Look at how the numbers change going down the left and right columns. There's a pattern there--see it?


Exponential Form

Expanded Form

Value

105

10  • 10 • 10 • 10 • 10

100,000

104

10  • 10 • 10 • 10

10,000

103

10  • 10 • 10

1,000

102

10 • 10

100

101

10

10


Moving down the table, each row drops one factor of 10 from the one above it. From row 1 to row 2, the exponential form goes from 105 to 104. The value drops from 100,000 to 10,000. Another way to put this is that each value is divided by 10 to produce the next value down the column.

Let's use this pattern of division by 10 to predict the value of 100.


Exponential Form

Expanded Form

Value

105

10  • 10 • 10 • 10 • 10

100,000

104

10  • 10 • 10 • 10

10,000

103

10  • 10 • 10

1,000

102

10 • 10

100

101

10

10

100

1

1


Following the pattern, you see that 100 is equal to 1. Would the pattern hold for a different base? Say a base of 3?


Exponential Form

Expanded Form

Value

35

3 • • • • 3

243

34

3  3  3 • 3

81

33

• • 3

27

32

3 • 3

9

31

3

3

30

1

1


Yes! And the same pattern would hold true for any non-zero number or variable raised to a power of 0, n0 = 1.


There is a conflict when the base is 0. You know that 03 = 0, 02 = 0, and 01 = 0, so you would expect 00 to also be equal to 0. However, the above pattern says that any base raised to the power of 0 is 1, so this leads you to believe that 00 = 1. Notice the competing patterns--00cannot be both 0 and 1! In this case, mathematicians say that the value of 00 is undefined. (And remember that undefined is not the same as 0!)


Exponents of 0 or 1

Any number or variable raised to a power of 1 is the number itself.  n1 = n

Any non-zero number or variable raised to a power of 0 is equal to 1. n0 = 1

The quantity 00 is undefined.


Example

Problem

Evaluate.

2x0 if x = 9

2 • 90

Substitute 9 for the variable x.

2 • 1

Evaluate 90.

2

Multiply.

Answer

2x2,if x = 9


As done previously, to evaluate expressions containing exponents of 0 or 1, substitute the value of the variable into the expression and simplify.


Evaluate the expression 3x0 - y1, if x = 12 and y = -6.

 

A) 42

B) 3

C) 9

D) 2


Negative Exponents

What does it mean when an exponent is a negative integer? Let's use the powers of 10 pattern from earlier to find out. If you continue this pattern to add some more rows, beyond 100, you find the following:


Exponential Form

Expanded Form

Value

105

10  • 10 • 10 • 10 • 10

100,000

104

10  • 10 • 10 • 10

10,000

103

10  • 10 • 10

1,000

102

10 • 10

100

101

10

10

100

1

1

10-1

10-2


Following the pattern, you see that 100 is equal to 1. Then you get into negative exponents: 10-1 is equal to , and 10-2 isthe same as .

Following this pattern, a number with a negative exponent can be rewritten as the reciprocal of the original number, with a positive exponent.

For example, 10-3 = and 10-7=.

To see if these patterns hold true for numbers other than 10, check out this table with powers of 3.


Exponential Form

Expanded Form

Value

35

3 • • • • 3

243

34

3  3  3 • 3

81

33

• • 3

27

32

3 • 3

9

31

3

3

30

1

1

3-1

3-2


The numbers are different but the patterns are the same. We are now ready to state the definition of a negative exponent.


Negative Exponent

For any non-zero number and any integer x, n-x. For example, 5-2.


Note that the definition above states that the base, n must be a "non-zero number.”


Evaluate the expression (x-2) • (x0) when x = 6.

 

A) 

B) 

C) 0

D) 36


Summary

Exponential notation is composed of a base and an exponent. It is a "shorthand” way of writing repeated multiplication, and indicates that the base is a factor and the exponent is the number of times the factor is used in the multiplication. The basic rules of exponents are as follows:


·An exponent applies only to the value to its immediate left.

·When a quantity in parentheses is raised to a power, the exponent applies to everything inside the parentheses.

·For any non-zero number nn0 = 1.

·For any non-zero number and any integer x, n -x .

Permissions

This reading is taken from the Developmental Math Open Program created by The NROC Project. It is available under a Creative Commons license. 

Modifié le: jeudi 25 août 2016, 09:41