3.7: Negative Exponents
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Overview
- Reciprocals
- Negative Exponents
- Working with Negative Exponents
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.
4⋅14=1. This means that 4 and 14 are reciprocals.
6⋅16=1. This means that 6 and 16 are reciprocals.
−2⋅−12=1. This means that −2 and −12 are reciprocals.
a⋅1a=1. This means that a and 1a are reciprocals.
x⋅1x=1. This means that x and 1x are reciprocals.
x3⋅1x3=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 x−3. Assume x≠0.
x3⋅x−3=x3+(−3)=x0=1
Thus, since the product of x3 and x−3 is 1, x3 and x−3 must be reciprocals.
We also know that x3⋅1x3=1. (See problem 6 above.) Thus, x3 and 1x3 are also reciprocals.
Then, since x−3 and 1x3 are both reciprocals of x3 and a real number can have only one reciprocal, it must be that x−3=1x3.
We have used −3 as the exponent, but the process works as well for all other negative integers. We make the following definition.:
If n is any natural number and x is any nonzero real number, then:
x−n=1xn
Sample Set A
Write each of the following so that only positive exponents appear.
x−6=1x6
a−1=1aa=1a
7−2=172=149
(3a)−6=1(3a)6
(5x−1)−24=1(5x−1)−24
(k+2x)−(−8)=(k+2z)8
Practice Set A
Write each of the following using only positive exponents.
y−5
- Answer
-
1y5
m−2
- Answer
-
1m2
3−2
- Answer
-
19
5−1
- Answer
-
15
2−4
- Answer
-
116
(xy)−4
- Answer
-
1(xy)4
(a+2b)−12
- Answer
-
1(a+2b)12
(m−n)−(−4)
- Answer
-
(m−n)4
It is important to note that a−n is not necessarily a negative number. For example,
3−2=132=19 3−2≠ −9
Working with Negative Exponents
The problems of Sample Set A suggest the following rule for working with exponents:
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.
x−2y5.
The factor x−2 can be moved from the numerator to the denominator by changing the exponent −2 to +2
x−2y5=y5x2
a9b−3.
The factor b−3 can be moved from the numerator to the denominator by changing the exponent −3 to +3.
a9b−3=a9b3
a4b2c−6.
This fraction can be written without any negative exponents by moving the factor c−6 into the numerator.
We must change the −6 to +6 to make the move legitimate.
a4b2c−6=a4b2c6
1x−3y−2z−1.
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.
1x−3y−2z−1=x3y2z1=x3y2z
Practice Set B
Write each of the following so that only positive exponents appear.
x−4y7
- Answer
-
y7x4
a2b−4
- Answer
-
a2b4
x3y4z−8
- Answer
-
x3y4z8
6m−3n−27k−1
- Answer
-
6k7m3n2
1a−2b−6c−8
- Answer
-
a2b6c8
3a(a−5b)−25b(a−4b)5
- Answer
-
3a5b(a−5b)2(a−4b)5
Sample Set C
Rewrite 24a7b923a4b−6 in a simpler form.
Notice that we are dividing powers with the same base. We'll proceed by using the rules of exponents.
24a7b923a4b−6=24a7b98a4b−6=3a7−4b9−(−6)=3a3b9+6=3a3b15
Write 9a5b35x3y2 so that no denominator appears.
We can eliminate the denominator by moving all factors that make up the denominator to the numerator.
9a5b35−1x−3y−2
Find the value of 110−2+34−3
We can evaluate this expression by eliminating the negative exponents.
110−2+34−3=1⋅102+3⋅43=1⋅100+3⋅64=100+192=292
Practice Set C
Rewrite 36x8b332x−2b−5 in a simpler form.
- Answer
-
4x10b8
Write 24m−3n74−1x5 in a simpler form and one in which no denominator appears.
- Answer
-
64m−3n7x−5
Find the value of 25−2+6−2⋅23⋅32
- Answer
-
52
Exercises
Write the following expressions using only positive exponents. Assume all variables are nonzero.
x−2
- Answer
-
1x−2
x−4
x−7
- Answer
-
1x7
a−8
a−10
- Answer
-
1a−10
b−12
b−14
- Answer
-
1b14
y−1
y−5
- Answer
-
1y5
(x+1)−2
(x−5)−3
- Answer
-
1(x−5)3
(y−4)−6
(a+9)−10
- Answer
-
1(a+9)10
(r+3)−8
(a−1)−12
- Answer
-
1(a−1)12
x3y−2
x7y−5
- Answer
-
x7y5
a4b−1
a7b−8
- Answer
-
a7b8
a2b3c−2
x3y2z−6
- Answer
-
x3y2z6
x3y−4z2w
a7b−9zw3
- Answer
-
a7zw3b9
a3b−1zw2
x5y−5z−2
- Answer
-
x5y5z2
x4y−8z−3w−4
a−4b−6c−1d4
- Answer
-
d4a4b6c
x9y−6z−1w−5r−2
4x−6y2
- Answer
-
4y2x6
5x2y2z−5
7a−2b2c2
- Answer
-
7b2c2a2
4x3(x+1)2y−4z−1
7a2(a−4)3b−6c−7
- Answer
-
7a2(a−4)3b6c7
18b−6(b2−3)−5c−4d5e−1
7(w+2)−2(w+1)3
- Answer
-
7(w+1)3(w+2)2
2(a−8)−3(a−2)5
(x2+3)3(x2−1)−4
- Answer
-
(x2+3)3(x2−1)4
(x4+2x−1)−6(x+5)4
(3x2−4x−8)−9(2x+11)−3
- Answer
-
1(3x2−4x−8)9(2x+11)3
(5y2+8y−6)−2(6y−1)−7
7a(a2−4)−2(b2−1)−2
- Answer
-
7a(a2−4)2(b2−1)2
(x−5)−43b2c4(x+6)8
(y3+1)−15y3z−4w−2(y3−1)−2
- Answer
-
5y3(y3+1)z4w2(y3−1)2
5x3(2x−7)
3y−3(9x)
- Answer
-
27xy3
6a−4(2a−6)
4a2b2a−5b−2
- Answer
-
4a3
5−1a−2b−6b−11c−3c9
23x22−3x−2
- Answer
-
1
7a−3b−9⋅5a6bc−2c4
(x+5)2(x+5)−6
- Answer
-
1(x+5)4
(a−4)3(a−4)−10
8(b+2)−8(b+2)−4(b+2)3
- Answer
-
8(b+2)9
3a5b−7(a2+4)−36a−4b(a2+4)−1(a2+4)
−4a3b−5(2a2b7c−2)
- Answer
-
−8a5b2c2
−2x−2y−4z4(−6x3y−3z)
(−5)2(−5)−1
- Answer
-
−5
(−9)−3(9)3
(−1)−1(−1)−1
- Answer
-
1
(4)2(2)−4
1a−4
- Answer
-
a4
1a−1
4x−6
- Answer
-
4x6
7x−8
23y−1
- Answer
-
23y
6a2b−4
3c5a3b−3
- Answer
-
3b3c5a3
16a−2b−6c2yz−5w−4
24y2z−86a2b−1c−9d3
- Answer
-
4bc9y2a2d3z8
3−1b5(b+7)−49−1a−4(a+7)2
36a6b5c832a3b7c9
- Answer
-
4a3b2c
45a4b2c615a2b7c8
33x4y3z32xy5z5
- Answer
-
3x3y2z4
21x2y2z5w47xyz12w14
33a−4b−711a3b−2
- Answer
-
3a7b5
51x−5y−33xy
26x−5y−2a−7b52−1x−4y−2b6
- Answer
-
128a7bx
(x+3)3(y−6)4(x+3)5(y−6)−8
4x3y7
- Answer
-
4x3y7
5x4y3a3
23a4b5c−2x−6y5
- Answer
-
23a4b5x6c2y5
23b5c2d−94b4cx
10x3y−73x5z2
- Answer
-
103x2y7z2
3x2y−2(x−5)9−1(x+5)3
14a2b2c−12(a2+21)−44−2a2b−1(a+6)3
- Answer
-
224b3c12(a2+21)4(a+6)3
For the following problems, evaluate each numerical expression.
4−1
7−1
- Answer
-
17
6−2
2−5
- Answer
-
132
3−4
6⋅3−3
- Answer
-
29
4⋅9−2
28⋅14−1
- Answer
-
2
2−3(3−2)
2−1⋅3−1⋅4−1
- Answer
-
124
10−2+3(10−2)
(−3)−2
- Answer
-
19
(−10)−1
32−3
- Answer
-
24
4−15−2
24−74−1
- Answer
-
36
2−1+4−12−2+4−2
210−262⋅6−13
- Answer
-
63
For the following problems, write each expression so that only positive exponents appear.
(a6)−2
(a5)−3
- Answer
-
1a15
(x7)−4
(x4)−8
- Answer
-
1x32
(b−2)7
(b−4)−1
- Answer
-
b4
(y−3)−4
(y−9)−3
- Answer
-
y27
(a−1)−1
(b−1)−1
- Answer
-
b
(a0)−1, a≠0
(m))−1, m≠0
- Answer
-
1
(x−3y7)−4
(x6y6z−1)2
- Answer
-
x12y12z2
(a−5b−1c0)6
(y3x−4)5
- Answer
-
x20y15
(a−8b−6)3
(2ab3)4
- Answer
-
16a4b12
(3ba2)−5
(5−1a3b−6x−2y9)2
- Answer
-
a6x425b12y18
(4m−3n62m−5n)3
(r5s−4m−8n7)−4
- Answer
-
n28s16m32r20
(h−2j−6k−4p)−5
Exercises for Review
Simplify (4x5y3z0)3
- Answer
-
64x15y9
Find the sum. −15+3
Find the difference. 8−(−12)
- Answer
-
20
Simplify (−3)(−8)+4(−5)
Find the value of m if m=−3k−5tkt+6 when k=4 and t=−2
- Answer
-
1