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Mathematics LibreTexts

3.7: Negative Exponents

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Overview

  • Reciprocals
  • Negative Exponents
  • Working with Negative Exponents

Reciprocals

Reciprocals

Two real numbers are said to be reciprocals of each other if their product is 1. Every nonzero real number has exactly one reciprocal, as shown in the examples below. Zero has no reciprocal.

Example 3.7.1

414=1. This means that 4 and 14 are reciprocals.

Example 3.7.2

616=1. This means that 6 and 16 are reciprocals.

Example 3.7.3

212=1. This means that 2 and 12 are reciprocals.

Example 3.7.4

a1a=1. This means that a and 1a are reciprocals.

Example 3.7.5

x1x=1. This means that x and 1x are reciprocals.

Example 3.7.6

x31x3=1. This means that x3 and 1x3 are reciprocals.

Negative Exponents

We can use the idea of reciprocals to find a meaning for negative exponents.

Consider the product of x3 and x3. Assume x0.

x3x3=x3+(3)=x0=1

Thus, since the product of x3 and x3 is 1, x3 and x3 must be reciprocals.

We also know that x31x3=1. (See problem 6 above.) Thus, x3 and 1x3 are also reciprocals.

Then, since x3 and 1x3 are both reciprocals of x3 and a real number can have only one reciprocal, it must be that x3=1x3.

We have used 3 as the exponent, but the process works as well for all other negative integers. We make the following definition.:

Negative Exponent Definition

If n is any natural number and x is any nonzero real number, then:

xn=1xn

Sample Set A

Write each of the following so that only positive exponents appear.

Example 3.7.7

x6=1x6

Example 3.7.8

a1=1aa=1a

Example 3.7.9

72=172=149

Example 3.7.10

(3a)6=1(3a)6

Example 3.7.11

(5x1)24=1(5x1)24

Example 3.7.12

(k+2x)(8)=(k+2z)8

Practice Set A

Write each of the following using only positive exponents.

Practice Problem 3.7.1

y5

Answer

1y5

Practice Problem 3.7.2

m2

Answer

1m2

Practice Problem 3.7.3

32

Answer

19

Practice Problem 3.7.4

51

Answer

15

Practice Problem 3.7.5

24

Answer

116

Practice Problem 3.7.6

(xy)4

Answer

1(xy)4

Practice Problem 3.7.7

(a+2b)12

Answer

1(a+2b)12

Practice Problem 3.7.8

(mn)(4)

Answer

(mn)4

Note

It is important to note that an is not necessarily a negative number. For example,

32=132=19 32 9

Working with Negative Exponents

The problems of Sample Set A suggest the following rule for working with exponents:

Moving Factors Up and Down

In a fraction, a factor can be moved from the numerator to the denominator or from the denominator to the numerator by changing the sign of the exponent.

Sample Set B

Write each of the following so that only positive exponents appear.

Example 3.7.13

x2y5.

The factor x2 can be moved from the numerator to the denominator by changing the exponent 2 to +2

x2y5=y5x2

Example 3.7.14

a9b3.

The factor b3 can be moved from the numerator to the denominator by changing the exponent 3 to +3.

a9b3=a9b3

Example 3.7.15

a4b2c6.

This fraction can be written without any negative exponents by moving the factor c6 into the numerator.
We must change the 6 to +6 to make the move legitimate.

a4b2c6=a4b2c6

Example 3.7.16

1x3y2z1.

This fraction can be written without negative exponents by moving all the factors from the denominator to the numerator. Change the sign of each exponent: 3 to +3, 2 to +2, 1 to +1.

1x3y2z1=x3y2z1=x3y2z

Practice Set B

Write each of the following so that only positive exponents appear.

Practice Problem 3.7.9

x4y7

Answer

y7x4

Practice Problem 3.7.10

a2b4

Answer

a2b4

Practice Problem 3.7.11

x3y4z8

Answer

x3y4z8

Practice Problem 3.7.12

6m3n27k1

Answer

6k7m3n2

Practice Problem 3.7.13

1a2b6c8

Answer

a2b6c8

Practice Problem 3.7.14

3a(a5b)25b(a4b)5

Answer

3a5b(a5b)2(a4b)5

Sample Set C

Example 3.7.17

Rewrite 24a7b923a4b6 in a simpler form.
Notice that we are dividing powers with the same base. We'll proceed by using the rules of exponents.
24a7b923a4b6=24a7b98a4b6=3a74b9(6)=3a3b9+6=3a3b15

Example 3.7.17

Write 9a5b35x3y2 so that no denominator appears.
We can eliminate the denominator by moving all factors that make up the denominator to the numerator.
9a5b351x3y2

Example 3.7.17

Find the value of 1102+343
We can evaluate this expression by eliminating the negative exponents.
1102+343=1102+343=1100+364=100+192=292

Practice Set C

Practice Problem 3.7.15

Rewrite 36x8b332x2b5 in a simpler form.

Answer

4x10b8

Practice Problem 3.7.16

Write 24m3n741x5 in a simpler form and one in which no denominator appears.

Answer

64m3n7x5

Practice Problem 3.7.17

Find the value of 252+622332

Answer

52

Exercises

Write the following expressions using only positive exponents. Assume all variables are nonzero.

Exercise 3.7.1

x2

Answer

1x2

Exercise 3.7.2

x4

Exercise 3.7.3

x7

Answer

1x7

Exercise 3.7.4

a8

Exercise 3.7.5

a10

Answer

1a10

Exercise 3.7.6

b12

Exercise 3.7.7

b14

Answer

1b14

Exercise 3.7.8

y1

Exercise 3.7.9

y5

Answer

1y5

Exercise 3.7.10

(x+1)2

Exercise 3.7.11

(x5)3

Answer

1(x5)3

Exercise 3.7.12

(y4)6

Exercise 3.7.13

(a+9)10

Answer

1(a+9)10

Exercise 3.7.14

(r+3)8

Exercise 3.7.15

(a1)12

Answer

1(a1)12

Exercise 3.7.16

x3y2

Exercise 3.7.17

x7y5

Answer

x7y5

Exercise 3.7.18

a4b1

Exercise 3.7.19

a7b8

Answer

a7b8

Exercise 3.7.20

a2b3c2

Exercise 3.7.21

x3y2z6

Answer

x3y2z6

Exercise 3.7.22

x3y4z2w

Exercise 3.7.23

a7b9zw3

Answer

a7zw3b9

Exercise 3.7.24

a3b1zw2

Exercise 3.7.25

x5y5z2

Answer

x5y5z2

Exercise 3.7.26

x4y8z3w4

Exercise 3.7.27

a4b6c1d4

Answer

d4a4b6c

Exercise 3.7.28

x9y6z1w5r2

Exercise 3.7.29

4x6y2

Answer

4y2x6

Exercise 3.7.30

5x2y2z5

Exercise 3.7.31

7a2b2c2

Answer

7b2c2a2

Exercise 3.7.32

4x3(x+1)2y4z1

Exercise 3.7.33

7a2(a4)3b6c7

Answer

7a2(a4)3b6c7

Exercise 3.7.34

18b6(b23)5c4d5e1

Exercise 3.7.35

7(w+2)2(w+1)3

Answer

7(w+1)3(w+2)2

Exercise 3.7.36

2(a8)3(a2)5

Exercise 3.7.37

(x2+3)3(x21)4

Answer

(x2+3)3(x21)4

Exercise 3.7.38

(x4+2x1)6(x+5)4

Exercise 3.7.39

(3x24x8)9(2x+11)3

Answer

1(3x24x8)9(2x+11)3

Exercise 3.7.40

(5y2+8y6)2(6y1)7

Exercise 3.7.41

7a(a24)2(b21)2

Answer

7a(a24)2(b21)2

Exercise 3.7.42

(x5)43b2c4(x+6)8

Exercise 3.7.43

(y3+1)15y3z4w2(y31)2

Answer

5y3(y3+1)z4w2(y31)2

Exercise 3.7.44

5x3(2x7)

Exercise 3.7.45

3y3(9x)

Answer

27xy3

Exercise 3.7.46

6a4(2a6)

Exercise 3.7.47

4a2b2a5b2

Answer

4a3

Exercise 3.7.48

51a2b6b11c3c9

Exercise 3.7.49

23x223x2

Answer

1

Exercise 3.7.50

7a3b95a6bc2c4

Exercise 3.7.51

(x+5)2(x+5)6

Answer

1(x+5)4

Exercise 3.7.52

(a4)3(a4)10

Exercise 3.7.53

8(b+2)8(b+2)4(b+2)3

Answer

8(b+2)9

Exercise 3.7.54

3a5b7(a2+4)36a4b(a2+4)1(a2+4)

Exercise 3.7.55

4a3b5(2a2b7c2)

Answer

8a5b2c2

Exercise 3.7.56

2x2y4z4(6x3y3z)

Exercise 3.7.57

(5)2(5)1

Answer

5

Exercise 3.7.58

(9)3(9)3

Exercise 3.7.59

(1)1(1)1

Answer

1

Exercise 3.7.60

(4)2(2)4

Exercise 3.7.61

1a4

Answer

a4

Exercise 3.7.62

1a1

Exercise 3.7.63

4x6

Answer

4x6

Exercise 3.7.64

7x8

Exercise 3.7.65

23y1

Answer

23y

Exercise 3.7.66

6a2b4

Exercise 3.7.67

3c5a3b3

Answer

3b3c5a3

Exercise 3.7.68

16a2b6c2yz5w4

Exercise 3.7.69

24y2z86a2b1c9d3

Answer

4bc9y2a2d3z8

Exercise 3.7.70

31b5(b+7)491a4(a+7)2

Exercise 3.7.71

36a6b5c832a3b7c9

Answer

4a3b2c

Exercise 3.7.72

45a4b2c615a2b7c8

Exercise 3.7.73

33x4y3z32xy5z5

Answer

3x3y2z4

Exercise 3.7.74

21x2y2z5w47xyz12w14

Exercise 3.7.75

33a4b711a3b2

Answer

3a7b5

Exercise 3.7.76

51x5y33xy

Exercise 3.7.77

26x5y2a7b521x4y2b6

Answer

128a7bx

Exercise 3.7.78

(x+3)3(y6)4(x+3)5(y6)8

Exercise 3.7.79

4x3y7

Answer

4x3y7

Exercise 3.7.80

5x4y3a3

Exercise 3.7.81

23a4b5c2x6y5

Answer

23a4b5x6c2y5

Exercise 3.7.82

23b5c2d94b4cx

Exercise 3.7.83

10x3y73x5z2

Answer

103x2y7z2

Exercise 3.7.84

3x2y2(x5)91(x+5)3

Exercise 3.7.85

14a2b2c12(a2+21)442a2b1(a+6)3

Answer

224b3c12(a2+21)4(a+6)3

For the following problems, evaluate each numerical expression.

Exercise 3.7.86

41

Exercise 3.7.87

71

Answer

17

Exercise 3.7.88

62

Exercise 3.7.89

25

Answer

132

Exercise 3.7.90

34

Exercise 3.7.91

633

Answer

29

Exercise 3.7.92

492

Exercise 3.7.93

28141

Answer

2

Exercise 3.7.94

23(32)

Exercise 3.7.95

213141

Answer

124

Exercise 3.7.96

102+3(102)

Exercise 3.7.97

(3)2

Answer

19

Exercise 3.7.98

(10)1

Exercise 3.7.99

323

Answer

24

Exercise 3.7.100

4152

Exercise 3.7.101

24741

Answer

36

Exercise 3.7.102

21+4122+42

Exercise 3.7.103

210262613

Answer

63

For the following problems, write each expression so that only positive exponents appear.

Exercise 3.7.104

(a6)2

Exercise 3.7.105

(a5)3

Answer

1a15

Exercise 3.7.106

(x7)4

Exercise 3.7.107

(x4)8

Answer

1x32

Exercise 3.7.108

(b2)7

Exercise 3.7.109

(b4)1

Answer

b4

Exercise 3.7.110

(y3)4

Exercise 3.7.111

(y9)3

Answer

y27

Exercise 3.7.112

(a1)1

Exercise 3.7.113

(b1)1

Answer

b

Exercise 3.7.114

(a0)1, a0

Exercise 3.7.115

(m))1, m0

Answer

1

Exercise 3.7.116

(x3y7)4

Exercise 3.7.117

(x6y6z1)2

Answer

x12y12z2

Exercise 3.7.118

(a5b1c0)6

Exercise 3.7.119

(y3x4)5

Answer

x20y15

Exercise 3.7.120

(a8b6)3

Exercise 3.7.121

(2ab3)4

Answer

16a4b12

Exercise 3.7.122

(3ba2)5

Exercise 3.7.123

(51a3b6x2y9)2

Answer

a6x425b12y18

Exercise 3.7.124

(4m3n62m5n)3

Exercise 3.7.125

(r5s4m8n7)4

Answer

n28s16m32r20

Exercise 3.7.126

(h2j6k4p)5

Exercises for Review

Exercise 3.7.127

Simplify (4x5y3z0)3

Answer

64x15y9

Exercise 3.7.128

Find the sum. 15+3

Exercise 3.7.129

Find the difference. 8(12)

Answer

20

Exercise 3.7.130

Simplify (3)(8)+4(5)

Exercise 3.7.131

Find the value of m if m=3k5tkt+6 when k=4 and t=2

Answer

1


This page titled 3.7: Negative Exponents is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by Denny Burzynski & Wade Ellis, Jr. (OpenStax CNX) .

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