5.7 Simplify and Use Square Roots

The topics covered in this section are:

1. Simplify expressions with square roots
2. Estimate square roots
3. Approximate square roots
4. Simplify variable expressions with square roots
5. Use square roots in applications

5.7.1 Simplify Expressions with Square Roots

To start this section, we need to review some important vocabulary and notation.

Remember that when a number $n$ is multiplied by itself, we can write this as $n^{2}$, which we read aloud as “$n$ squared.” For example, $8^{2}$ is read as “$8$ squared.”

We call $64$ the square of $8$ because $8^{2}=64$. Similarly, $121$ is the square of $11$, because $11^{2}=121$.

SQUARE OF A NUMBER

If $n^{2}=m$, then $m$ is the square of $n$.

Modeling Squares

Do you know why we use the word square? If we construct a square with three tiles on each side, the total number of tiles would be nine.

This is why we say that the square of three is nine.

$3^{2}=9$

The number $9$ is called a perfect square because it is the square of a whole number.

The chart shows the squares of the counting numbers $1$ through $15$. You can refer to it to help you identify the perfect squares.

PERFECT SQUARES

A perfect square is the square of a whole number.

What happens when you square a negative number?

$(-8)^{2} = (-8)(-8)$

$=64$

When we multiply two negative numbers, the product is always positive. So, the square of a negative number is always positive.

The chart shows the squares of the negative integers from $-1$ to $-15$.

Did you notice that these squares are the same as the squares of the positive numbers?

Square Roots

Sometimes we will need to look at the relationship between numbers and their squares in reverse. Because $10^{2} = 100$, we say $100$ is the square of $10$. We can also say that $10$ is a square root of $100$.

SQUARE ROOT OF A NUMBER

A number whose square is $m$ is called a square root of $m$.

If $n^{2}$, then $n$ is a square root of $m$.

Notice $(-10)^{2} = 100$ also, so $-10$ is also a square root of $100$. Therefore, both $10$ and $-10$ are square roots of $100$.

So, every positive number has two square roots: one positive and one negative.

What if we only want the positive square root of a positive number? The radical sign, $\sqrt{\ }$, stands for the positive square root. The positive square root is also called the principal square root.

SQUARE ROOT NOTATION

$\sqrt{m}$ is read as “the square root of $m$.”

If $m=m^{2}$, then $\sqrt{m}=n$ for $n \geq 0$.

We can also use the radical sign for the square root of zero. Because $0^{2} = 0$, $\sqrt{0} = 0$. Notice that zero has only one square root.

The chart shows the square roots of the first $15$ perfect square numbers.

Example 1

Simplify:

1. $\sqrt{25}$
2. $\sqrt{121}$

Every positive number has two square roots and the radical sign indicates the positive one. We write $\sqrt{100} = 10$. If we want to find the negative square root of a number, we place a negative in front of the radical sign. For example, $- \sqrt{100} = -10$.

Example 2

Simplify:

1. $- \sqrt{9}$
2. $- \sqrt{144}$

Square Root of a Negative Number

Can we simplify $\sqrt{-25}$? Is there a number whose square is $-25$?

$(\ \ )^{2} = -25$?

None of the numbers that we have dealt with so far have a square that is $-25$. Why? Any positive number squared is positive, and any negative number squared is also positive. In the next chapter we will see that all the numbers we work with are called the real numbers. So we say there is no real number equal to $\sqrt{-25}$. If we are asked to find the square root of any negative number, we say that the solution is not a real number.

Example 3

Simplify:

1. $\sqrt{-169}$
2. $- \sqrt{121}$

Square Roots and the Order of Operations

When using the order of operations to simplify an expression that has square roots, we treat the radical sign as a grouping symbol. We simplify any expressions under the radical sign before performing other operations.

Example 4

Simplify:

1. $\sqrt{25} + \sqrt{144}$
2. $\sqrt{25+144}$

Notice the different answers in Part 1 and Part 2 of Example 4. It is important to follow the order of operations correctly. In Part 1, we took each square root first and then added them. In Part 2, we added under the radical sign first and then found the square root.

5.7.2 Estimate Square Roots

So far we have only worked with square roots of perfect squares. The square roots of other numbers are not whole numbers.

We might conclude that the square roots of numbers between $4$ and $9$ will be between $2$ and $3$, and they will not be whole numbers. Based on the pattern in the table above, we could say that $\sqrt{5}$ is between $2$ and $3$3. Using inequality symbols, we write

$2< \sqrt{5} <3$

Example 5

Estimate $\sqrt{60}$ between two consecutive whole numbers.

5.7.3 Approximate Square Roots with a Calculator

There are mathematical methods to approximate square roots, but it is much more convenient to use a calculator to find square roots. Find the $\sqrt{\ \ }$ or $\sqrt{x}$ key on your calculator. You will to use this key to approximate square roots. When you use your calculator to find the square root of a number that is not a perfect square, the answer that you see is not the exact number. It is an approximation, to the number of digits shown on your calculator’s display. The symbol for an approximation is $\approx$ and it is read approximately.

Suppose your calculator has a $10$-digit display. Using it to find the square root of $5$ will give $2.236067977$. This is the approximate square root of $5$. When we report the answer, we should use the “approximately equal to” sign instead of an equal sign.

$\sqrt{5} \approx 2.236067978$

You will seldom use this many digits for applications in algebra. So, if you wanted to round $\sqrt{5}$ to two decimal places, you would write

$\sqrt{5} \approx 2.24$

How do we know these values are approximations and not the exact values? Look at what happens when we square them.

$2.236067978^{2} = 5.000000002$

$2.24^{2} = 5.0176$

The squares are close, but not exactly equal, to $5$.

Example 6

Round $\sqrt{17}$ to two decimal places using a claculator.

5.7.4 Simplify Variable Expressions with Square Roots

Expressions with square root that we have looked at so far have not had any variables. What happens when we have to find a square root of a variable expression?

Consider $\sqrt{9x^{2}}$, where $x \geq 0$. Can you think of an expression whose square is $9x^{2}$?

$(?)^{2} = 9x^{2}$

$(3x)^{2} = 9x^{2}$, so, $\sqrt{9x^{2}} = 3x$

When we use a variable in a square root expression, for our work, we will assume that the variable represents a non-negative number. In every example and exercise that follows, each variable in a square root expression is greater than or equal to zero.

Example 7

Simplify: $\sqrt{x^{2}}$.

Example 8

Simplify: $\sqrt{16x^{2}}$.

Example 9

Simplify: $- \sqrt{81y^{2}}$.

Example 10

Simplify: $\sqrt{36x^{2} y^{2}}$.

5.7.5 Use Square Roots in Applications

As you progress through your college courses, you’ll encounter several applications of square roots. Once again, if we use our strategy for applications, it will give us a plan for finding the answer!

HOW TO: Use a strategy for applications with square roots.

1. Identify what you are asked to find.
2. Write a phrase that gives the information to find it.
3. Translate the phrase to an expression.
4. Simplify the expression.
5. Write a complete sentence that answers the question.

Square Roots and Area

We have solved applications with area before. If we were given the length of the sides of a square, we could find its area by squaring the length of its sides. Now we can find the length of the sides of a square if we are given the area, by finding the square root of the area.

If the area of the square is $A$ square units, the length of a side is $\sqrt{A}$ units. See the table below.

Area (square units)	Length of side (units)
$9$	$\sqrt{9} = 3$
$144$	$\sqrt{144} = 12$
$A$	$\sqrt{A}$

Example 11

Mike and Lychelle want to make a square patio. They have enough concrete for an area of $200$ square feet. To the nearest tenth of a foot, how long can a side of their square patio be?

Square Roots and Gravity

Another application of square roots involves gravity. On Earth, if an object is dropped from a height of $h$ feet, the time in seconds it will take to reach the ground is found by evaluating the expression$\frac{\sqrt{h}}{4}$. For example, if an object is dropped from a height of $64$ feet, we can find the time it takes to reach the ground by evaluating $\frac{\sqrt{64}}{4}$.

	$\frac{\sqrt{64}}{4}$
Take the square root of $64$.	$\frac{8}{4}$
Simplify the fraction.	$2$

It would take $2$ seconds for an object dropped from a height of $64$ feet to reach the ground.

Example 12

Christy dropped her sunglasses from a bridge $400$ feet above a river. How many seconds does it take for the sunglasses to reach the river?

Square Roots and Accident Investigations

Police officers investigating car accidents measure the length of the skid marks on the pavement. Then they use square roots to determine the speed, in miles per hour, a car was going before applying the brakes. According to some formulas, if the length of the skid marks is $d$ feet, then the speed of the car can be found by evaluating $\sqrt{24d}$.

Example 13

After a car accident, the skid marks for one car measured $190$ feet. To the nearest tenth, what was the speed of the car (in mph) before the brakes were applied?