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Elementary Algebra

9.1 Simplify and Use Square Roots

Elementary Algebra9.1 Simplify and Use Square Roots

Learning Objectives

By the end of this section, you will be able to:

  • Simplify expressions with square roots
  • Estimate square roots
  • Approximate square roots
  • Simplify variable expressions with square roots

Be Prepared 9.1

Before you get started, take this readiness quiz.

  1. Simplify: 9292 (−9)2(−9)2 9292.
    If you missed this problem, review Example 1.50.
  2. Round 3.846 to the nearest hundredth.
    If you missed this problem, review Example 1.94.
  3. For each number, identify whether it is a real number or not a real number:
    100100 −100−100.
    If you missed this problem, review Example 1.113.

Simplify Expressions with Square Roots

Remember that when a number nn is multiplied by itself, we write n2n2 and read it “n squared.” For example, 152152 reads as “15 squared,” and 225 is called the square of 15, since 152=225152=225.

Square of a Number

If n2=mn2=m, then mm is the square of nn.

Sometimes we will need to look at the relationship between numbers and their squares in reverse. Because 225 is the square of 15, we can also say that 15 is a square root of 225. A number whose square is mm is called a square root of mm.

Square Root of a Number

If n2=mn2=m, then nn is a square root of mm.

Notice (−15)2=225(−15)2=225 also, so −15−15 is also a square root of 225. Therefore, both 15 and −15−15 are square roots of 225.

So, every positive number has two square roots—one positive and one negative. What if we only wanted the positive square root of a positive number? The radical sign, mm, denotes the positive square root. The positive square root is also called the principal square root.

We also use the radical sign for the square root of zero. Because 02=002=0, 0=00=0. Notice that zero has only one square root.

Square Root Notation

This figure is a picture of an m inside a square root sign. The sign is labeled as a radical sign and the m is labeled as the radicand.

mm is read as “the square root of mm.”

If m=n2m=n2, then m=nm=n, for n0n0.

The square root of mm, mm, is the positive number whose square is mm.

Since 15 is the positive square root of 225, we write 225=15225=15. Fill in Figure 9.2 to make a table of square roots you can refer to as you work this chapter.

This table has fifteen columns and two rows. The first row contains the following numbers: the square root of 1, the square root of 4, the square root of 9, the square root of 16, the square root of 25, the square root of 36, the square root of 49, the square root of 64, the square root of 81, the square root of 100, the square root of 121, the square root of 144, the square root of 169, the square root of 196, and the square root of 225. The second row is completely empty except for the last column. The number 15 is in the last column.
Figure 9.2

We know that every positive number has two square roots and the radical sign indicates the positive one. We write 225=15225=15. If we want to find the negative square root of a number, we place a negative in front of the radical sign. For example, 225=−15225=−15.

Example 9.1

Simplify: 3636 196196 8181 289289.

Try It 9.1

Simplify: 4949 225225.

Try It 9.2

Simplify: 6464 121121.

Example 9.2

Simplify: −169−169 6464.

Try It 9.3

Simplify: −196−196 8181.

Try It 9.4

Simplify: 4949 −121−121.

When using the order of operations to simplify an expression that has square roots, we treat the radical as a grouping symbol.

Example 9.3

Simplify: 25+14425+144 25+14425+144.

Try It 9.5

Simplify: 9+169+16 9+169+16.

Try It 9.6

Simplify: 64+22564+225 64+22564+225.

Estimate Square Roots

So far we have only considered square roots of perfect square numbers. The square roots of other numbers are not whole numbers. Look at Table 9.1 below.

Number Square Root
4 44 = 2
5 55
6 66
7 77
8 88
9 99 = 3
Table 9.1

The square roots of numbers between 4 and 9 must be between the two consecutive whole numbers 2 and 3, and they are not whole numbers. Based on the pattern in the table above, we could say that 55 must be between 2 and 3. Using inequality symbols, we write:

2<5<32<5<3

Example 9.4

Estimate 6060 between two consecutive whole numbers.

Try It 9.7

Estimate the square root 3838 between two consecutive whole numbers.

Try It 9.8

Estimate the square root 8484 between two consecutive whole numbers.

Approximate Square Roots

There are mathematical methods to approximate square roots, but nowadays most people use a calculator to find them. Find the xx key on your calculator. You will 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 square root. It is an approximation, accurate to the number of digits shown on your calculator’s display. The symbol for an approximation is and it is read ‘approximately.’

Suppose your calculator has a 10-digit display. You would see that

52.23606797852.236067978

If we wanted to round 55 to two decimal places, we would say

52.2452.24

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(2.236067978)2=5.000000002(2.24)2=5.0176

Their squares are close to 5, but are not exactly equal to 5.

Using the square root key on a calculator and then rounding to two decimal places, we can find:

4=252.2462.4572.6582.839=34=252.2462.4572.6582.839=3

Example 9.5

Round 1717 to two decimal places.

Try It 9.9

Round 1111 to two decimal places.

Try It 9.10

Round 1313 to two decimal places.

Simplify Variable Expressions with Square Roots

What if we have to find a square root of an expression with a variable? Consider 9x29x2. Can you think of an expression whose square is 9x29x2?

(?)2=9x2(3x)2=9x2,so9x2=3x(?)2=9x2(3x)2=9x2,so9x2=3x

When we use the radical sign to take the square root of a variable expression, we should specify that x0x0 to make sure we get the principal square root.

However, in this chapter we will assume that each variable in a square-root expression represents a non-negative number and so we will not write x0x0 next to every radical.

What about square roots of higher powers of variables? Think about the Power Property of Exponents we used in Chapter 6.

(am)n=am·n(am)n=am·n

If we square amam, the exponent will become 2m2m.

(am)2=a2m(am)2=a2m

How does this help us take square roots? Let’s look at a few:

25u8=5u4because(5u4)2=25u816r20=4r10because(4r10)2=16r20196q36=14q18because(14q18)2=196q3625u8=5u4because(5u4)2=25u816r20=4r10because(4r10)2=16r20196q36=14q18because(14q18)2=196q36

Example 9.6

Simplify: x6x6 y16y16.

Try It 9.11

Simplify: y8y8 z12z12.

Try It 9.12

Simplify: m4m4 b10b10.

Example 9.7

Simplify: 16n216n2.

Try It 9.13

Simplify: 64x264x2.

Try It 9.14

Simplify: 169y2169y2.

Example 9.8

Simplify: 81c281c2.

Try It 9.15

Simplify: 121y2121y2.

Try It 9.16

Simplify: 100p2100p2.

Example 9.9

Simplify: 36x2y236x2y2.

Try It 9.17

Simplify: 100a2b2100a2b2.

Try It 9.18

Simplify: 225m2n2225m2n2.

Example 9.10

Simplify: 64p6464p64.

Try It 9.19

Simplify: 49x3049x30.

Try It 9.20

Simplify: 81w3681w36.

Example 9.11

Simplify: 121a6b8121a6b8

Try It 9.21

Simplify: 169x10y14169x10y14.

Try It 9.22

Simplify: 144p12q20144p12q20.

Media

Access this online resource for additional instruction and practice with square roots.

Section 9.1 Exercises

Practice Makes Perfect

Simplify Expressions with Square Roots

In the following exercises, simplify.

1.

36 36

2.

4 4

3.

64 64

4.

169 169

5.

9 9

6.

16 16

7.

100 100

8.

144 144

9.

4 4

10.

100 100

11.

1 1

12.

121 121

13.

−121 −121

14.

−36 −36

15.

−9 −9

16.

−49 −49

17.

9 + 16 9 + 16

18.

25 + 144 25 + 144

19.

9 + 16 9 + 16

20.

25 + 144 25 + 144

Estimate Square Roots

In the following exercises, estimate each square root between two consecutive whole numbers.

21.

70 70

22.

55 55

23.

200 200

24.

172 172

Approximate Square Roots

In the following exercises, approximate each square root and round to two decimal places.

25.

19 19

26.

21 21

27.

53 53

28.

47 47

Simplify Variable Expressions with Square Roots

In the following exercises, simplify.

29.

y 2 y 2

30.

b 2 b 2

31.

a 14 a 14

32.

w 24 w 24

33.

49 x 2 49 x 2

34.

100 y 2 100 y 2

35.

121 m 20 121 m 20

36.

25 h 44 25 h 44

37.

81 x 36 81 x 36

38.

144 z 84 144 z 84

39.

81 x 18 81 x 18

40.

100 m 32 100 m 32

41.

64 a 2 64 a 2

42.

25 x 2 25 x 2

43.

144 x 2 y 2 144 x 2 y 2

44.

196 a 2 b 2 196 a 2 b 2

45.

169 w 8 y 10 169 w 8 y 10

46.

81 p 24 q 6 81 p 24 q 6

47.

9 c 8 d 12 9 c 8 d 12

48.

36 r 6 s 20 36 r 6 s 20

Everyday Math

49.

Decorating Denise wants to have a square accent of designer tiles in her new shower. She can afford to buy 625 square centimeters of the designer tiles. How long can a side of the accent be?

50.

Decorating Morris wants to have a square mosaic inlaid in his new patio. His budget allows for 2025 square inch tiles. How long can a side of the mosaic be?

Writing Exercises

51.

Why is there no real number equal to −64−64?

52.

What is the difference between 9292 and 99?

Self Check

After completing the exercises, use this checklist to evaluate your mastery of the objectives of this section.

This table has four columns and five rows. The columns are labeled, “I can…,” “Confidentally,” “With some help,” and “No – I don’t get it!” Under the “I can…,” column are, “simplify expressions with square roots.,” “estimate square roots.,” “approximate square roots.,” and “4) simplify variable expressions with square roots.” All the other rows under the different columns are empty.

On a scale of 1–10, how would you rate your mastery of this section in light of your responses on the checklist? How can you improve this?

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