Definitions

Using an exponent is a way of expressing multiplication of a number by itself. The exponent says how many times the number, called the base, is used as a factor. An expression that has a base and exponent is called a power.

Examples

52 = 5 × 5 = 25

base = 5, exponent = 2

63 = 6 × 6 × 6 = 216

base = 6, exponent = 3

34 = 3 × 3 × 3 × 3 = 81

base = 3, exponent = 4

Two exponents have special names. Numbers with the exponent 2 are called squared.  The expression 52 can be read as five to the second power or five squared.

Numbers with the exponent 3 are called cubed. The expression 63 can be read as six to the third power or six cubed.

The expression 34 is read as three to the fourth power.

The reciprocal for a number with a positive exponent, 1/an (n is a positive integer), is equivalent to the number with the negative exponent, a-n.

a-n = 1/an

Rules of Exponents

This list outlines the rules of exponents and shows an example of each.

1)

a0 = 1, a ≠ 0

50 = 1

A number to the zero power equals 1. 00 is undefined.

2)

a1 = a

71 = 7

A number to the first power is equal to itself.

3)

am × an = am + n

32 × 33 = 32 + 3 = 35
x4 × x2 = x4 + 2 = x6

Add the exponents when multiplying two terms with the same base.

4)

am/an = am – n, a ≠ 0

75/73 = 75 – 3 = 72
y6/y4 = y6 – 4 = y2

Subtract exponents when dividing two terms that have the same base.

5)

(am)n = amn

(23)2 = 23 × 2 = 26
(y4)3 = y4 × 3 = y12

Multiply the exponents to raise a power to a power.

6)

(ab)n = anbn

(3 × 5)2 = 32 52
(x2y)3 = (x2)3y3 = x6y3

To find the power of a product, distribute the power to each factor.

7)

(a/b)n = an/bn, b ≠ 0

(3/4)3 = 33/43
(x5/y)2=(x5)2/y2=x10/y2

The power of a quotient distributes the power to the numerator and denominator.

8)

an = 1/a-n

52 = 1/5-2

A number with a positive exponent can be written as the reciprocal for a negative exponent.

Frequent Mistakes

Remember to use Experiment as a technique if you’re not sure you remember a rule. For example, here are some common mistakes shown with numbers used to test the rules.

an + am ≠ an + m

24 + 23 is not equal to 27.

16 + 8 ≠ 128

an – am ≠ an – m

24 – 23 is not equal to 2.

16 – 8 ≠ 2

an + bn ≠ (a + b)n

32 + 52 is not equal to (3 + 5)2.

9 + 25 ≠ 64

an + an ≠ a2n

72 + 72 is not equal to 74.

49 + 49 ≠ 49 × 49

an + an + an ≠ a3n

23 + 23 + 23 is not equal to 29.

8 + 8 + 8 ≠ 512

an × am ≠ anm

22 × 23 is not equal to 26.

4 × 8 ≠ 64

Exponents and Negative Numbers

When applying exponents to negative numbers, make sure your answer has the correct sign.

A negative number taken to an even exponent is always a positive number.

(-2)4 = (-2) × (-2) × (-2) × (-2) = 16
(-5)-2 = 1/(-5)2 = 1/((-5) × (-5)) = 1/25

A negative number taken to an odd exponent is always a negative number.

(-4)3 = (-4) × (-4) × (-4) = -64
(-2)-3 = 1/(-2)3 = 1/( (-2) × (-2) × (-2)) = 1/(-8) = -1/8

But a leading negative sign is not affected by the exponent.

-52 = -(25) = -25
-(-4)3 = -((-4) × (-4) × (-4)) = -(-64) = 64

Fractional Exponents

When a base is a non-negative number, it can be raised to a fractional power. Fractional exponents are the link between roots and exponents.  When the exponent is a fraction, the numerator says what power to raise the base to, and the denominator says what root to take.  You can raise the base to the power or take the root in either order.

Fractional exponents will be covered more completely in the next section, Roots.

When the numerator of the fractional exponent is 1, take the nth root of the base.

a1/n  = na

The exponent 1/2 means take the square root of the base.

a1/2 = √a

91/2 = √9 = 3

The exponent 1/3 means take the cube root of the base.

a1/3 = 3a

1251/3 = 3√125 = 35 × 5 × 5 = 5

The exponent m/n means raise the base to the power m and take the nth root. Note, that m can be any integer, while n is a positive integer only.

am/n = nam

82/3 = 382 = 3(23)2 = 326 = 22 = 4

Simplifying Exponential Expressions

Looking at the answers before starting to solve the problem is a good strategy for questions with exponents.  You may match the answer format by just simplifying or estimating, rather than calculating to get a value.

Simplifying may be all that is needed. Answers will often have exponents rather than a numerical value.

An estimate needs to match the accuracy of the possible answer choices.

For any answer format, the first step is often to factor then simplify using the rules of exponents.

Remember to use Experiment as a technique if you’re not sure you remember a rule.

Example

242 × 153 =

Solution

Answers will often contain the exponents rather than multiplying to get a value. Find the prime factors before multiplying the terms.
242 × 153 = (2 × 2 × 2 × 3)2 × (3 × 5)3 = (23 × 3)2 × (3 × 5)3 = (23)2 × 32 × 33 × 53 = 26 × 35 × 53

Example

Evaluate:  82/45

Solution

Find the prime factors to get the same base, then use the rule am/an = am – n.
82/45 = (23)2/(22)5 = 26/210 = 26 – 10 = 2-4

To compare to the answer choices, you can evaluate further, to get a fraction or a decimal.
2-4 = 1/24 = 1/16 = 0.0625

Example

Simplify:  [(2xy3 × 9y)] / [(3xy)2]

Solution

Remember that on the GMAT, fractions might be on one line. So rewrite this as “built up”, then simplify.

Do the multiplication, then use the rule am/an = am – n for each base. (This is often called cancellation.)
(2xy3 × 9y) / (3xy)2 = 18xy4/9x2y2 = 2y2/x

Example

Evaluate: 5/3-2

Solution

Apply the rule a-n = 1/an to the denominator.
5/3-2 = 5/1/32  Remember that division by a fraction is multiplication by its reciprocal.
5/1/32 = 5 × 32 = 45

Example

Factor.

  1. 72 + 75
  2. 32 + 122

Solution

Factor these by “undoing” using the distributive property.

  1. 72 + 75 = 72(1 + 73)
  2. 32 + 122 = 32 + (3 × 4)2 = 32 + 32 42 = 32(1 + 42)
Example

If x = 52 + 53 + 54 + 55, what is the greatest prime factor of x?

Solution

Factor to find all the prime factors.

52 + 53 + 54 + 55
= 52(1 + 5 + 52 + 53)
= 52 (1 + 5 + 25 + 125)
= 52 (156)
= 52 (2 × 2 × 3 × 13)

So the greatest prime factor is 13.

Example

A microorganism has a length of 5-7 meters.  If 625 microorganisms of this length are arranged in a line, what is the length of the line in millimeters? (1m = 100 cm, 1 cm = 10 mm)

Solution

Factor 625.
625 = 252 = (52)2 = 54

Multiply.  Use the rule am × an = am + n.
(5-7)(54) = 5-3 = 1/53 = 13/53 = (1/5) × (1/5) × (1/5) = 0.2 × 0.2 × 0.2 = 0.008

Convert from meters to millimeters. 1 m = 100 cm, 1 cm = 10 mm, therefore 1 m = 1,000 mm (or 1 mm = 10-3 m), so multiply both sides by 1,000.
0.008 m = 8 mm

Common Values

These powers commonly appear on the GMAT, so they should be memorized.

Squares

22 = 4
32 = 9
42 = 16
52 = 25
62 = 36
72 = 49
82 = 64
92 = 81
102 = 100
112 = 121
122 = 144
132 = 169
142 = 196
152 = 225

Cubes

23 = 8
33 = 27
43 = 64
53 = 125
63 = 216
103 = 1,000

Other Powers

20 = 1
21 = 2
24 = 16
34 = 81
54 = 625
104 = 10,000
25 = 32
35 = 243
26 = 64
27 = 128
28 = 256
29 = 512
210 = 1,024

Knowing these values and the rules of exponents means you can more quickly calculate other values.

For example, since 210 = 1,024, you know 212 = 1,024 × 4 = 4,096 and 2-10 = 1/1,024

Rules of Exponents

This list outlines the rules of exponents and shows an example of each.

1)

a0  = 1, a ≠ 0

50  = 1

A number to the zero power equals 1. 00 is undefined.

2)

a1 = a

71 = 7

A number to the first power is equal to itself.

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