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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CK-12 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/square-root-algorithm.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 , we say that is the square root of .

**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 is written as or sometimes as . The symbol is sometimes called a **radical sign**.

Numbers with whole-number 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) , so .

b) , so .

c) , so .

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 , where is the product of half the paired factors we pulled out and is the product of the leftover factors.

#### Example B

Find the principal square root of the following numbers.

a) 8

b) 48

c) 75

**Solution**

a) . This gives us one pair of 2’s and one leftover 2, so .

b) , so , or .

c) , so .

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.

#### Example C

Simplify the following square root problems

a)

b)

c)

d)

**Solution**

a)

b)

c)

d)

**Approximate Square Roots**

Terms like and (square roots of prime numbers) cannot be written as **rational numbers**. That is to say, they cannot be expressed as the ratio of two integers. We call them **irrational numbers**. In decimal form, they have an unending, seemingly random, string of numbers after the decimal point.

To find approximate values for square roots, we use the or button on a calculator. When the number we plug in is a perfect square, or the square of a rational number, we will get an exact answer. When the number is a non-perfect square, the answer will be irrational and will look like a random string of digits. Since the calculator can only show some of the infinitely many digits that are actually in the answer, it is really showing us an **approximate answer**—not exactly the right answer, but as close as it can get.

#### 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.

CK-12 Foundation: Square Roots

### Guided Practice

*Find the square root of each number.*

a) 576

b) 216

**Solution**

a) , so .

b) , so , or .

### Explore More

For 1-10, find the following square roots **exactly without using a calculator**, giving your answer in the simplest form.

- (Hint: The division rules you learned can be applied backwards!)

For 11-20, use a calculator to find the following square roots. Round to two decimal places.

### Answers for Explore More Problems

To view the Explore More answers, open this PDF file and look for section 2.9.