Number

YOU ARE LEARNING:

Indices: Negative Numbers

We can have negative numbers as the base number and must be able to calculate indices with these.

An index number is a base number raised to a power. How do you write $6$ squared?

We can also raise negative numbers to a power. What is the correct way of writing minus $3$ squared?

Brackets are needed to be clear that we are squaring the negative number.

Without brackets, we square the number first and then make negative.

What is another way of writing $(-3)^2$ ?

What is $(-3)^2$ ?

Work out $(-6)^2$

There is a pattern when squaring negative numbers. What type of number is the answer?

The square of a negative number is always positive.

For example $(-4)^2 = (-4) \times (-4) = 16$ .

However, this isn't the case when cubing negative numbers.

Let's find $(-3)^3$

Cubing multiplies a number by itself three times

So $(-3)^3$ is the same as $(-3) \times (-3) \times (-3)$

What is $(-3) \times (-3)$ ?

What is $9 \times (-3)$ ?

Cubing negative numbers always returns a negative

This is true for every negative number. Nice 👍

What is $(-5)^3$ ?

We have seen that

Squaring a negative number gives a positive answer Cubing a negative number gives a negative answer

What about other powers? What is $(-3)^4$ ?

The answer to $(-3)^4$ was positive. Will the answer to $(-3)^5$ be positive or negative?

We have a pattern here.

Negative base number + EVEN power = Positive number Negative base number + ODD power = Negative number

Is $(-143)^{21}$ a positive or negative number?

Summary!

We can raise a negative number to a power too.

$(-5)^2$ for example

Use brackets to show that the negative is part of the number (we calculate indices first in BIDMAS).

$(-5)^2=(-5)\times (-5)=25$

$-5^2=-(5\times 5)=-25$

When the base number is negative, the power will determine whether the answer is positive or negative.

Negative base number + EVEN power = Positive number Negative base number + ODD power = Negative number