Index Notation

Index Notation

A convenient way to write a product of $\textit{identical factors}$ is to use $\textbf{exponential}$ or $\textbf{index notation}$.
Rather than writing $5 \times 5 \times 5 \times 5$, we can write this product as $5^4$.

The small $4$ is called the $\textbf{exponent}$ or $\textbf{index}$, and the $5$ is called the $\textbf{base}$.
If $n$ is a positive integer, then $a^n$ is the product of $n$ factors of $a$.
$$ \large a^n=\overbrace {a \times a \times a \times \cdots \times a}^{n} $$
We say that $a$ is the $\textbf{base}$, and $n$ is the $\textbf{exponent}$ or $\textbf{index}$.

The following table shows the first four powers of $2$.

$$ \large \begin{array}{|c|c|c|c|} \hline \textit{Natural number} & \textit{Factorised form} & \textit{Exponent form} & \textit{Spoken form} \\ \hline
2 & 2 & 2^1 & \text{two} \\ \hline
4 & 2 \times 2 & 2^2 & \text{two squared} \\ \hline
8 & 2 \times 2 \times 2 & 2^3 & \text{two cubed} \\ \hline
16 & 2 \times 2 \times 2 \times 2 & 2^4 & \text{two to the fourth} \\ \hline
\end{array} $$

Any non-zero number raised to the power zero is equal to $1$.
$$ \large a^0 = 1, a \ne 0$$
$0^0$ is undefined.

$\textbf{Negative Bases}$

\( \begin{eqnarray}
(-1)^1 &=& -1 \\
(-1)^2 &=& -1 \times -1 = 1 \\
(-1)^3 &=& -1 \times -1 \times -1 = -1 \\
(-1)^4 &=& -1 \times -1 \times -1 \times -1 = 1 \\
(-2)^1 &=& -2 \\
(-2)^2 &=& -2 \times -2 = 4 \\
(-2)^3 &=& -2 \times -2 \times -2 = -8 \\
(-2)^4 &=& -2 \times -2 \times -2 \times -2 = 16 \\
\end{eqnarray} \)

From the patterns above, we can see that:

$\textbf{negative}$ base raised to an $\textbf{even}$ exponent is $\textbf{positive}$.

$\textbf{negative}$ base raised to an $\textbf{odd}$ exponent is $\textbf{negative}$.

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Example 1

List the first four powers of 3.

$3^1 = 3$
$3^2 = 3 \times 3 = 9$
$3^3 = 3 \times 3 \times 3 = 27$
$3^4 = 3 \times 3 \times 2 \times 3 = 81$

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Example 2

Simplify $(-1)^{17}$.

$(-1)^{17} = -1$

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Example 3

Simplify $(-1)^{18}$.

$(-1)^{18} = 1$

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Example 4

Simplify $5^0$.

$5^0 = 1$

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