Albert Teen

YOU ARE LEARNING:

Indices: Negative Powers

# Indices: Negative Powers

### Indices: Negative Powers

Raising a number to a negative power creates a reciprocal: a fraction where 1 is the denominator.

1

The power in an index number can be negative like $3^{-2}$ or $4^{-5}$

To see how this works, we need to divide some index numbers.

2

What is $\dfrac{x^3}{x^5}$?

3

We have found that $\dfrac{x^3}{x^5}=x^{-2}$, an answer which has a negative power.

But what does $x^{-2}$ mean?

1

We found that $\dfrac{x^3}{x^5}=x^{-2}$.

Let's expand our calculation to understand the answer.

2

What is another way of writing $x^3$?

3

We can write $x^5=x\times x\times x\times x\times x$

This gives $\dfrac{x^3}{x^5}=\dfrac{x\times x\times x}{x\times x\times x\times x\times x}$.

4

We can now cancel the $x$ pairs in the numerator and denominator.

$\dfrac{{\cancel{x}}\times {\cancel{x}}\times {\cancel{x}}}{{\cancel{x}}\times {\cancel{x}}\times {\cancel{x}}\times x\times x}$

5

Now we have cancelled the $x$ terms, how is this simplified? $\dfrac{{\cancel{x}}\times {\cancel{x}}\times {\cancel{x}}}{{\cancel{x}}\times {\cancel{x}}\times {\cancel{x}}\times x\times x}$

6

By breaking the fraction down we have shown that

$\dfrac{x^3}{x^5}=\dfrac{1}{x^2}$

7

We have seen that $\dfrac{x^3}{x^5}=x^{-2}$ and $\dfrac{x^3}{x^5}=\dfrac{1}{x^2}$, what does that mean?

8

This is the rule for negative powers

$x^{-2}=\dfrac{1}{x^2}$

1

We have found the rule for negative powers, $x^{-2}=\dfrac{1}{x^2}$. How can we state this in words?

2

A reciprocal is one over the original number.

The reciprocal of $2$ is $\dfrac{1}{2}$.

1

If the power is negative, express it as a reciprocal

Have a look at the image

2

Let's try this one $8^{-2}$.

First we take out the negative sign to give $8^2$.

3

First, what is the reciprocal of $8^2$?

4

Finally, we calculate the denominator to give

$8^{-2}=\dfrac{1}{64}$.

What is $6^{-1}$?

Let's have a go at an example: $3^{-3}$

1

Remove the negative sign

This will leave us with the index $3^3$.

2

Find the reciprocal of $3^3$.

3

So far we have $3^{-3}=\dfrac{1}{3^3}$, what is the final answer?

4

Our final fractions is

$\dfrac{1}{27}$ Nice 👍

Try this one: what is $4^{-3}$?

What is $5^{-2}$?

1

Summary! A reciprocal is one over the original number

The reciprocal of $2$ is $\dfrac{1}{2}$.

2

Indices or powers can be negative numbers

$x^{-y}$ is an index number with a negative power.

3

A negative power is the reciprocal of the positive power

The image shows how that works

4

We can find $2^{-2}$

First we have $2^{-2}=\dfrac{1}{2^2}$

We simplify to $\dfrac{1}{2^2}=\dfrac{1}{4}=0.25$