2.9: Square Roots and Irrational Numbers
What if you had a number like 1000 and you wanted to find its square root? After completing this concept, you'll be able to find square roots like this one by hand and with a calculator.
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CK12 Foundation: 0209S Square Roots (H264)
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You can also work out square roots by hand using a method similar to long division. (See the web page at http://www.homeschoolmath.net/teaching/squarerootalgorithm.php for an explanation of this method.)
Guidance
The square root of a number is a number which, when multiplied by itself, gives the original number. In other words, if
Note: Negative numbers and positive numbers both yield positive numbers when squared, so each positive number has both a positive and a negative square root. (For example, 3 and 3 can both be squared to yield 9.) The positive square root of a number is called the principal square root.
The square root of a number
Numbers with wholenumber square roots are called perfect squares. The first five perfect squares (1, 4, 9, 16, and 25) are shown below.
You can determine whether a number is a perfect square by looking at its prime factors. If every number in the factor tree appears an even number of times, the number is a perfect square. To find the square root of that number, simply take one of each pair of matching factors and multiply them together.
Example A
Find the principal square root of each of these perfect squares.
a) 121
b) 225
c) 324
Solution
a)
b)
c)
For more practice matching numbers with their square roots, try the Flash games at http://www.quia.com/jg/65631.html.
When the prime factors don’t pair up neatly, we “factor out” the ones that do pair up and leave the rest under a radical sign. We write the answer as
Example B
Find the principal square root of the following numbers.
a) 8
b) 48
c) 75
Solution
a)
b)
c)
Note that in the last example we collected the paired factors first, then we collected the unpaired ones under a single radical symbol. Here are the four rules that govern how we treat square roots.

a√×b√=ab−−√ 
Aa√×Bb√=ABab−−√ 
a√b√=ab−−√ 
Aa√Bb√=ABab−−√
Example C
Simplify the following square root problems
a)
b)
c)
d)
Solution
a)
b)
c)
d)
Approximate Square Roots
Terms like
To find approximate values for square roots, we use the
Example D
Use a calculator to find the following square roots. Round your answer to three decimal places.
a)
b)
c)
d)
Solution
a)
b)
c)
d)
Watch this video for help with the Examples above.
CK12 Foundation: Square Roots
Vocabulary
 The square root of a number is a number which gives the original number when multiplied by itself. In algebraic terms, for two numbers
a andb , ifa=b2 , then \begin{align*}b = \sqrt{a}\end{align*}.  A square root can have two possible values: a positive value called the principal square root, and a negative value (the opposite of the positive value).
 A perfect square is a number whose square root is an integer.
 Some mathematical properties of square roots are:
 \begin{align*}\sqrt{a} \ \times \sqrt{b} = \sqrt{ab}\end{align*}
 \begin{align*}A \sqrt{a} \ \times B \sqrt{b} = AB \sqrt{ab}\end{align*}
 \begin{align*}\frac{\sqrt{a}}{\sqrt{b}} = \sqrt{ \frac{a}{b}}\end{align*}
 \begin{align*}\frac{A\sqrt{a}}{B \sqrt{b}} = \frac{A}{B} \sqrt{\frac{a}{b}}\end{align*}
 Square roots of numbers that are not perfect squares (or ratios of perfect squares) are irrational numbers. They cannot be written as rational numbers (the ratio of two integers). In decimal form, they have an unending, seemingly random, string of numbers after the decimal point.
 Computing a square root on a calculator will produce an approximate solution since the calculator only shows a finite number of digits after the decimal point.
Guided Practice
Find the square root of each number.
a) 576
b) 216
Solution
a) \begin{align*}576 = (2 \times 2) \times (2 \times 2) \times (2 \times 2) \times (3 \times 3)\end{align*}, so \begin{align*}\sqrt{576} = 2 \times 2 \times 2 \times 3 = 24\end{align*}.
b) \begin{align*}216 = (2 \times 2) \times 2 \times (3 \times 3) \times 3\end{align*}, so \begin{align*}\sqrt{216} = 2 \times 3 \times \sqrt{2 \times 3}\end{align*}, or \begin{align*} 6 \sqrt{6}\end{align*}.
Practice
For 110, find the following square roots exactly without using a calculator, giving your answer in the simplest form.
 \begin{align*}\sqrt{25}\end{align*}
 \begin{align*}\sqrt{24}\end{align*}
 \begin{align*}\sqrt{20}\end{align*}
 \begin{align*}\sqrt{200}\end{align*}
 \begin{align*}\sqrt{2000}\end{align*}
 \begin{align*}\sqrt{\frac{1}{4}}\end{align*} (Hint: The division rules you learned can be applied backwards!)
 \begin{align*}\sqrt{\frac{9}{4}}\end{align*}
 \begin{align*}\sqrt{0.16}\end{align*}
 \begin{align*}\sqrt{0.1}\end{align*}
 \begin{align*}\sqrt{0.01}\end{align*}
For 1120, use a calculator to find the following square roots. Round to two decimal places.
 \begin{align*}\sqrt{13}\end{align*}
 \begin{align*}\sqrt{99}\end{align*}
 \begin{align*}\sqrt{123}\end{align*}
 \begin{align*}\sqrt{2}\end{align*}
 \begin{align*}\sqrt{2000}\end{align*}
 \begin{align*}\sqrt{.25}\end{align*}
 \begin{align*}\sqrt{1.35}\end{align*}
 \begin{align*}\sqrt{0.37}\end{align*}
 \begin{align*}\sqrt{0.7}\end{align*}
 \begin{align*}\sqrt{0.01}\end{align*}
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Term  Definition 

approximate solution  An approximate solution to a problem is a solution that has been rounded to a limited number of digits. 
Irrational Number  An irrational number is a number that can not be expressed exactly as the quotient of two integers. 
Perfect Square  A perfect square is a number whose square root is an integer. 
principal square root  The principal square root is the positive square root of a number, to distinguish it from the negative value. 3 is the principal square root of 9; 3 is also a square root of 9, but it is not principal square root. 
rational number  A rational number is a number that can be expressed as the quotient of two integers, with the denominator not equal to zero. 
Square Root  The square root of a term is a value that must be multiplied by itself to equal the specified term. The square root of 9 is 3, since 3 * 3 = 9. 
Image Attributions
Here you'll learn how to find and approximate square roots. You'll also learn how to simplify expressions involving square roots.