7.09 Multiplying and dividing with indices

When multiplying a number by itself repeatedly, we are able to use index notation to write the expression more simply. Here we are going to look at a rule that allows us simplify products that involve the multiplication of index terms.

Consider the expression a^{5} \times a^{3}. Notice that the terms share like bases

Let's think about what this would look like if we distributed the expression: a^{5} \times a^{3}=(a\times a\times a\times a\times a)\times (a\times a\times a)

We can see that there are eight a's being multiplied together, and notice that 8 is the sum of the powers in the original expression.

So, in our example above, \begin{aligned}a^{5}\times a^{3} &= a^{5+3}\\&=a^{8}\end{aligned}

Let's look at a numeric example. Say we wanted to find the value of 4^{2} \times 4^{3}. By evaluating each product separately we would have\begin{aligned}4^{2}\times 4^{3} &= 16\times64\\&=1024\end{aligned}

Alternatively, by first expanding the terms in the original expression we can arrive at a simplified version of the expression on our way to the final value.\begin{aligned}4^{2}\times 4^{3} &= 4\times4\times4\times4\times4\\&=4^{5}\\&=1024\end{aligned}

Notice in the second line we have identified that 4^{2} \times 4^{3}=4^{5}.

We can avoid having to write each expression in expanded form by using the multiplication law.

For any base number a, and any numbers m and n as powers, a^{m} \times a^{n}=a^{m+n}

That is, when multiplying terms with a common base:

• Keep the same base

• Find the sum of the powers

The multiplication law only works for terms with the same bases.

Consider the expression 7^{2} \times 3^{2}.

7 and 3 are not the same base terms, so we cannot simplify this expression any further.

But we can simplify the following expression: 7^{2} \times 3^{2} in another way.

Notice that both terms have the same index, so we can multiply their bases, and keep the same index.\begin{aligned}7^{2}\times 3^{2} &= 7\times7\times3\times3 \\ &= (7\times3)\times(7\times3) \\ &=21^{2}\end{aligned}

For any base numbers a and b that both have n as a power, a^{n} \times b^{n}=(a \times b)^{n}

That is, when multiplying terms with the same index:

• Find the product of the bases

• Keep the same power

Examples

The method to divide power terms is similar to the multiplication law, however in this case we subtract the powers from one another, rather than add them. Let's look at an expanded example to see why this is the case.

If we wanted to simplify the expression a^{6} \div a^{2}, we could write it as:

We can see that there are six a's being divided by two a's to give a result of four a's, and notice that 4 is the difference of the powers in the original expression. So: \begin{aligned} a^{6} \div a^{2} &= a^{6-2} \\ &=a^4 \end{aligned}

Let's look at another specific example. Say we wanted to find the value of 2^{7} \div 2^{3}. By evaluating each term in the quotient separately we would have

Alternatively, by first distributing the terms in the original expression we can arrive at a simplified version of the expression on our way to the final value.

Notice in the second line we have identified that 2^{7}\div 2^{3}=2^{4}.

We can avoid having to write each expression in expanded form by using the division law (which is also known as the quotient law): \dfrac{a^{m}}{a^{n}}=a^{m-n}where a is any base number, and m and n as powers.

That is, when dividing terms with a common base:

• Keep the same base

• Find the difference in the power.

We can also write the division law in the form a^{m}\div a^{n} = a^{m-n}.

As with using the multiplication (or product) law, we can only apply the division (or quotient) law to terms with the same bases (just like we can only add and subtract like terms in algebra). We can simplify \dfrac{9^{8}}{9^{3}} because the numerator and denominator have the same base: 9.

We cannot simplify \dfrac{8^{5}}{7^{3}} because the two terms do not have the same base (one has a base of 8 and the other has a base of 7).

But like with multiplication we can simplify \dfrac{8^{5}}{2^{5}}by thinking of it as \left(\dfrac{8}{2}\right)^{5}=4^{5}.

For any base numbers a and b that both have n as a power,a^{n} \div b^{n}=(a \div b)^{n}

That is, when dividing terms with the same index:

• Find the quotient of the bases

• Keep the same power

Examples

What happens if we want to divide one term by another and when we perform the subtraction and we are left with a power of 0? For example,\begin{aligned}4^{5} \div 4^{5} &=4^{5-5} \\ &=4^{0}\end{aligned}

To think about what value we can assign to the term 4^{0}, let's write this division problem as the fraction \dfrac{4^{5}}{4^{5}}. Since the numerator and denominator are the same, the fraction simplifies to 1. Notice that this will also be the case with \dfrac{4^{20}}{4^{20}} or any expression where we are dividing like bases whose powers are the same.

So the result we arrive at by using index laws is 4^{0}, and the result we arrive at by simplifying fractions is 1. This must mean that 4^{0}=1.

There is nothing special about the number 4, so we can extend this observation to any other base. This result is summarised by the zero power law.

For any base number a,a^{0}=1This says that taking the zeroth power of any number will always result in 1.

Examples