Lesson Explainer: Zero and Negative Exponents | Nagwa Lesson Explainer: Zero and Negative Exponents | Nagwa

Lesson Explainer: Zero and Negative Exponents Mathematics • 8th Grade

In this explainer, we will learn how to simplify and evaluate expressions involving zero and negative indices.

Let us first recall what a power is.

Definition: Powers and Exponential Form

Powers are numbers resulting from a repeated multiplication of a factor. Their general form is 𝑏, where 𝑏 is called the base and 𝑚 the exponent.

The base 𝑏 is the factor repeatedly multiplied by itself, and the exponent 𝑚 is the number of times 𝑏 appears in the repeated multiplication.

When a number is written as a power, we say it is written in exponential form.

When a power is written as a repeated multiplication, we say it is written in expanded form.

We should also know the following four rules, or laws, of exponents that we will use in our working.

Rule: Product Rule

The product of two powers with the same base is a power of this same base with an exponent equal to the sum of the exponents: 𝑏𝑏=𝑏.

Rule: Quotient Rule

The quotient of two powers with the same base is a power of this same base with an exponent equal to the difference of the exponents of the dividend and divisor: 𝑏𝑏=𝑏.

Rule: Power of a Product Rule

A given power of a product of factors is the product of each factor raised to that given power: (𝑎𝑏)=𝑎𝑏.

Rule: Power of a Power Rule

Raising a power to a further power is done by writing the base with an exponent equal to the product of the two exponents: (𝑏)=𝑏.

Let us start with an example of division of powers of the same base.

Example 1: Quotient of Powers of the Same Base

Simplify 7×7×77×7×7×7.

Answer

We notice that in this expression, all the powers have the same base: 7. By expanding all the powers, we see that the numerator would consist of a repeated multiplication of the factor 7 and that the factor 7 would appear 3+2+6=11 times. This can be written as 7 . This is the product rule, which states that 𝑏𝑏=𝑏. Similarly, we find that the denominator is 7. We have already simplified our expression to 77.

Now, we know that one 7 in the numerator cancels out with one 7 in the denominator since 77=1. So, here, the eleven 7s in the numerator cancel out with eleven out of the twelve 7s in the denominator to give 1. This leaves us with 17=17.

This can be visualized below: 77=77777777777777777777777=17.

The question now is whether we could express the result of the previous question, 17, as a power of 7.

So far, we have assumed that the exponent is positive. To understand the meaning of extending the exponents to negative numbers, let us look at what happens when we divide the power of a number by this number, for instance, 7 divided by 7. Given that 7=777, we see that 7÷7=177=7.

If we perform the same division again on 7, we get 7÷7=17=7.

The number 7 is of course simply 7, but it is useful to write the exponent here, as you will see in the following.

So, we start seeing a pattern here: every time we divide by 7, the exponent decreases by 1.

If we carry out the division by 7 once more, we get 7÷7=1=7.

One further division by 7 leads to 7÷7=1÷7=7. and another one leads to 7÷7=1÷7÷7=7.

Let us summarize these findings in a table.

7177
717
71
71÷7=17
71÷7÷7=17

We have just found out some very interesting information, which leads us to two important rules. First, when the exponent is zero, the answer is 1. This result holds true for all nonzero bases.

Rule: Zero Exponent Rule

Any nonzero base with an exponent of zero is equal to 1: 𝑏=1.

A consequence of this rule is that whenever we see a nonzero number raised to the power of zero, we can always write down its value as 1 without doing any further calculations. In addition, note that by applying the quotient rule 𝑏𝑏=𝑏 in the case 𝑛=𝑚, we can derive the zero exponent rule directly: 𝑏=𝑏𝑏, which simplifies to 𝑏=1.

Now, we state the second rule. In the above working, when the exponent is positive, it tells us how many times the base is used in a repeated multiplication of 1. When the exponent is negative, it tells us how many times the base is used in a repeated division of 1. From this, it follows that any power of a number is the reciprocal of the power with the same base but with an opposite exponent. This is the negative exponent rule.

Rule: Negative Exponent Rule

Any power can be expressed as the reciprocal (or multiplicative inverse) of a power with the same base but opposite exponent: 𝑏=1𝑏.

Hence, we find that 17 is a power of 7 with an exponent of 1 (i.e., 17=7). In the first example, we found that 77=7, and if we remember how pairs of factors in the numerator and denominator cancel out to give 1, we can easily conclude that 77=7=7.

In the next example, let us practice how to handle powers with a negative exponent.

Example 2: Evaluating Powers with Negative Exponents

Evaluate the following expression: 62.

Answer

The given expression involves the multiplication of two powers that have a different base, both with a negative exponent.

Recall the negative exponent rule, which tells us that any power is the reciprocal of the power having the same base but an opposite exponent: 𝑏=1𝑏. Therefore, 6=16 and 2=12. In other words, a negative exponent tells us how many times the base appears in a repeated division of 1. Thus, 6=166 and 2=122, so we have 62=166122=13614=1144.

Note that as an alternative approach, we could have worked backward from the power of a product rule, which states that a given power of a product of factors is the product of each factor raised to that given power: (𝑎𝑏)=𝑎𝑏. Applied to our expression, we find that 62=(62)=12.

Then, applying the negative exponent rule 𝑏=1𝑏, we find that 12=112=1144.

Hence, we have found that 62=1144.

Let us look now at powers of fractions. For instance, let us consider 35.

Note that since the fraction is within parentheses, the base of the power is the whole fraction, in contrast to 35 where only the numerator is a power (it is 3 raised to the fourth power).

We can write 35 in expanded form to get 35×35×35×35.

This expression is a repeated multiplication of 35, which can also be seen as a repeated multiplication of 3 and a repeated multiplication of 15 or as a repeated multiplication of 3 and a repeated division of 5. Using the commutativity of multiplication, we can rewrite the above expression as 3×3×3×3×15×15×15×15 or as 3×3×3×35×5×5×5.

Note that we could have found the above expression using the rule for multiplying fractions.

Our last expression can be simplified to 35 or, using our knowledge of negative exponents, to 35.

A power of a fraction is therefore just a special case of a power of a product. Let us generalize our result.

Rule: Power of a Fraction Rule

Raising a fraction to a given power is done by raising its numerator and its denominator individually: 𝑎𝑏=𝑎𝑏=𝑎𝑏.

Now, we can try an example.

Example 3: Evaluating Negative Single-Term Rational Expressions with Negative Integer Exponents

Which of the following is equal to 34?

  1. 68
  2. 169
  3. 916
  4. 916
  5. 169

Answer

Here, we have a negative fraction with a negative integer exponent and we need to work out its value.

We start by transforming the negative exponent into a positive one. Recall the power of a power rule, which tells us that raising a power to a further power is done by writing the base with an exponent equal to the product of the two exponents: (𝑏)=𝑏.

By writing the exponent 2 as (1)2, we can work backward from this rule by taking 𝑏=34, 𝑚=1, and 𝑛=2, giving 34=34=34.()

Notice that inside the big parentheses, we have a fraction to the power of 1. Recall the negative exponent rule, which tells us that any power can be expressed as the reciprocal of a power with the same base but opposite exponent: 𝑏=1𝑏. Applying this to 34, we see that it is equivalent to 1. This is the reciprocal of the fraction 34, which can be found by swapping the numerator and denominator. Thus, we get 43. So, we have now simplified our original expression to 43.

To evaluate this, we can use the power of a fraction rule, which states that raising a fraction to a given power is done by raising its numerator and its denominator individually: 𝑎𝑏=𝑎𝑏.

We need, however, to be careful here because the fraction is negative. Therefore, we can rewrite 43 as (1)43 or 43 to make sure we do not forget the negative sign. Doing this and then applying the power of a fraction rule, we get 43=(1)43=(1)43=143=43.

Finally, we evaluate the numerator and denominator separately to get the answer 169.

We conclude that 34=169, so answer E is correct.

So far, we have combined our rules for zero and negative exponents with other rules of exponents to help us evaluate numerical expressions. Very usefully, it turns out that the same rules can also be applied to help us simplify algebraic expressions. Here is an example of this type.

Example 4: Simplifying an Algebraic Expression Involving Negative Powers

Simplify 𝑥𝑥.

Answer

In this question, we are given a quotient comprising two negative powers with the same base, 𝑥.

Recall the quotient rule, which states that the quotient of two powers with the same base is a power of this same base with an exponent equal to the difference of the exponents of the dividend and divisor: 𝑏𝑏=𝑏.

Here, we have 𝑏=𝑥, 𝑚=2, and 𝑛=4, so applying the rule gives 𝑥𝑥=𝑥=𝑥=𝑥.()()

Therefore, when we simplify 𝑥𝑥, we get 𝑥.

In our final two questions, the power of a power rule will be especially important in helping us simplify exponents. We start with a numerical example.

Example 5: Evaluating Positive Rational Expressions with Negative Integer Exponents Raised to Positive Integer Exponents

Which of the following expressions has the same value as 23?

  1. 23
  2. 32
  3. 32
  4. 32
  5. 23

Answer

The given expression is a negative power of the fraction 23 raised to a further power, which is positive.

Recall the power of a power rule, which tells us that raising a power to a further power is done by writing the base with an exponent equal to the product of the two exponents: (𝑏)=𝑏.

Therefore, we can apply this rule with 𝑏=23, 𝑚=3, and 𝑛=5, which gives 23=23=23.()

The original expression has now been transformed into a fraction base with a single negative exponent.

It will be helpful if we can find a way to make the exponent positive. Note that we can do this by once more applying the power of a power rule: (𝑏)=𝑏. By writing the exponent 15 as (1)15, we can work backward from this rule by taking 𝑏=23, 𝑚=1, and 𝑛=15, giving 23=23=23.()

Notice that inside the big parentheses, we have a fraction to the power of 1. Now, recall the negative exponent rule, which tells us that any power can be expressed as the reciprocal of a power with the same base but opposite exponent: 𝑏=1𝑏. Applying this to 23, we see that it is equivalent to 1. This is the reciprocal of the fraction 23, which can be found by swapping the numerator and denominator. Thus, we get 32, which means we have now simplified our original expression to 32.

Finally, we can use the power of a fraction rule, which states that raising a fraction to a given power is done by raising its numerator and its denominator individually: 𝑎𝑏=𝑎𝑏.

Here, we have 𝑎=3, 𝑏=2, and 𝑚=15, so 32=32.

This is now in the right form to compare with the given answer options, so we see that C is correct.

We finish with an example that involves simplifying an algebraic expression by using many of the exponent rules we have explored.

Example 6: Simplifying an Algebraic Expression Involving Negative Powers

Which of the following expressions has the same value as ?

  1. 𝑎𝑏
  2. 𝑎𝑏
  3. 𝑎𝑏
  4. 𝑎𝑏
  5. 𝑎𝑏

Answer

Here, we have a complicated fraction expression, which we will refer to as the main fraction, raised to the power of 7. Notice that this main fraction is made up of three powers of the same base, namely the algebraic fraction 𝑎𝑏. Our strategy will be to simplify the main fraction by applying some of the rules of exponents to these three powers.

Starting with the numerator 𝑎𝑏𝑎𝑏, we see that the first term has a zero exponent. Since the zero exponent rule tells us that any nonzero base with an exponent of zero is equal to 1, then the numerator simplifies as 𝑎𝑏𝑎𝑏=1𝑎𝑏=𝑎𝑏.

Alternatively, we could have applied the product rule 𝑥𝑥=𝑥 with 𝑥=𝑎𝑏, 𝑚=0, and 𝑛=2 to get the same result by a different method: 𝑎𝑏𝑎𝑏=𝑎𝑏=𝑎𝑏().

Therefore, having simplified the numerator, we can now rewrite the main fraction as .

We have thus obtained a quotient of two powers with the same base. Recalling the quotient rule 𝑥𝑥=𝑥 and taking 𝑥=𝑎𝑏, 𝑚=2, and 𝑛=5, the main fraction simplifies as follows: =𝑎𝑏=𝑎𝑏=𝑎𝑏.()()

The last stage of simplification is to raise the main fraction to the power of 7, which gives 𝑎𝑏.

Notice that this is now in the form of a power of a power of 𝑎𝑏, as are all the answer options.

Recall that the power of a power rule says that raising a power to a further power is done by writing the base with an exponent equal to the product of the two exponents: (𝑥)=𝑥.

If we apply this rule to our simplified expression by taking 𝑥=𝑎𝑏, 𝑚=3, and 𝑛=7, we get 𝑎𝑏=𝑎𝑏=𝑎𝑏.()

Therefore, the correct answer option will be the one where the product of its two exponents is equal to 21. Working through them in order, we get 73=21,7(7)=49,(7)5=35,(7)(7)=49,(7)3=21.

Since the final option gives a result of 21, we conclude that E is the correct answer.

Let us finish by recapping some key concepts from this explainer.

Key Points

  • Powers are numbers resulting from a repeated multiplication of a factor. Their general form is 𝑏, where 𝑏 is called the base and 𝑚 the exponent.
  • By using the laws of exponents, we can derive further ones for zero and negative exponents.
  • The zero exponent rule states that any nonzero base with an exponent of zero is equal to 1: 𝑏=1.
  • The negative exponent rule states that any power can be expressed as the reciprocal (or multiplicative inverse) of a power with the same base but opposite exponent: 𝑏=1𝑏.

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