UK Secondary (7-11) Simplify negative square and cube roots
Lesson

Finding the square root of a number is the inverse (or opposite) operation of squaring a number.

Remember squaring a number means multiplying a number by itself (e.g. $3^2=3\times3$32=3×3).  The square root of a number is the number  that you need to multiply by itself to get the original number. What? Let's look at an example:

#### Example

##### question 1

Evaluate: $\sqrt{25}$25

Think: $5^2=25$52=25

Do: $\sqrt{25}=5$25=5

## Square Root of a Negative Number

So what happens when we bring in negative numbers?

Firstly, let me point out at this stage, you will NOT have to find the square root of a negative number. Why?

Remember when we were squaring numbers- multiplying two negative numbers always gives a positive answer. So we never get a negative answer. The answer to a negative square root is called an imaginary number.

Let's look at questions where we can include a negative symbol.

#### Examples

##### Question 2

Evaluate: $-\sqrt{64}$64

Think: $8^2=64$82=64

Do: $-\sqrt{64}=-8$64=8

## Cubed Root of a Negative Number

We can find the cubed root of a negative number (because when we cube a negative number, we get a negative answer).

#### Examples

##### Question 3

Evaluate: $\sqrt{-8}$38

Think: $\left(-2\right)^3=-8$(2)3=8

Do: $\sqrt{-8}=-2$38=2

Here's another one:

##### question 4

Evaluate: $-\sqrt{-216}$3216

Think: $6^3=216$63=216

Do:

 $-\sqrt{-216}$−3√−216 $=$= $-1\times\left(-6\right)$−1×(−6) $=$= $6$6

## Questions Using Roots

Building on the concepts that you have learnt already, including how to add and subtract integers, how to multiply and divide integers, as well as order of operations, you can do more complex questions using negative square and cubed roots.

#### Examples

Evaluate: $\sqrt{-64}\times\sqrt{64}$364×64

Think: The cube root of $-64$64 is $-4$4 and the square root of $64$64 is $8$8.

Do:

 $\sqrt{-64}\times\sqrt{64}$3√−64×√64 $=$= $-4\times8$−4×8 $=$= $-32$−32

Here's another one

Evaluate: $\left(-\sqrt{85+15}\right)\times\left(-\sqrt{-125}\right)$(85+15)×(3125)

Think: The square root of $100$100 is $10$10. The cubed root of $-125$125 is $-5$5.

Do:

 $\left(-\sqrt{85+15}\right)\times\left(-\sqrt{-125}\right)$(−√85+15)×(−3√−125) $=$= $\left(-\sqrt{100}\right)\times\left(-1\right)\times\left(-5\right)$(−√100)×(−1)×(−5) $=$= $-10\times5$−10×5 $=$= $-50$−50

#### Worked Example Videos

##### Question 1

Evaluate $\sqrt{-64}$364

##### Question 2

Evaluate $\sqrt{-125}\times\sqrt{27}$3125×327