4.1 Place Value and Roundingmhhe.com/math/devmath/streeter/bms/graphics/streeter5bms/ch04/...4.1 Place Value and Rounding 4.1 OBJECTIVES 1. Identify place value in a ... The ones place
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1. Identify place value in a decimal fraction2. Write a decimal in words3. Write a decimal as a fraction or mixed number4. Compare the size of several decimals5. Round a decimal to the nearest tenth6. Round a decimal to any specified decimal place
In Chapter 3, we looked at common fractions. Let’s turn now to a special kind of fraction,a decimal fraction. A decimal fraction is a fraction whose denominator is a power of 10.
Some examples of decimal fractions are , , and .
Earlier we talked about the idea of place value. Recall that in our decimal place-valuesystem, each place has one-tenth the value of the place to its left.
123
1000
45
100
3
10
NOTE Remember that thepowers of 10 are 1, 10, 100,1000, and so on. You mightwant to review Section 1.7before going on.
NOTE The decimal pointseparates the whole-numberpart and the fractional part of adecimal fraction.
Example 1
Identifying Place Values
Label the place values for the number 538.
5 3 8
Hundreds Tens Ones
The ones place value is one-tenth of the tens placevalue; the tens place value is one-tenth of thehundreds place value; and so on.
C H E C K Y O U R S E L F 1
Label the place values for the number 2793.
We now want to extend this idea to the right of the ones place. Write a period to the rightof the ones place. This is called the decimal point. Each digit to the right of that decimalpoint will represent a fraction whose denominator is a power of 10. The first place to theright of the decimal point is the tenths place:
0.1 �1
10
Writing a Number in Decimal Form
Write the mixed number in decimal form.
Tenths
Ones The decimal point
32
10� 3.2
32
10
Example 2
292 CHAPTER 4 DECIMALS
As you move farther to the right, each place value must be one-tenth of the value before
it. The second place value is hundredths . The next place is thousandths, the
fourth position is the ten thousandths place, and so on. Figure 1 illustrates the value of eachposition as we move to the right of the decimal point.
What are the place values for 4 and 6 in the decimal 2.34567? The place value of 4 is hun-dredths, and the place value of 6 is ten thousandths.
C H E C K Y O U R S E L F 3
What is the place value of 5 in the decimal of Example 3?
Understanding place values will allow you to read and write decimals by using thefollowing steps.
Step 1 Read the digits to the left of the decimal point as a whole number.Step 2 Read the decimal point as the word “and.”Step 3 Read the digits to the right of the decimal point as a whole number
followed by the place value of the rightmost digit.
Step by Step: Reading or Writing Decimals in Words
NOTE For convenience we willshorten the term “decimalfraction” to “decimal” from thispoint on.
NOTE If there are no nonzerodigits to the left of the decimalpoint, start directly with step 3.
PLACE VALUE AND ROUNDING SECTION 4.1 293
Writing a Decimal Number in Words
Write each decimal number in words.
5.03 is read “five and three hundredths.”
Hundredths
12.057 is read “twelve and fifty-seven thousandths.”
Thousandths
0.5321 is read “five thousand three hundred twenty-one ten thousandths.”
The rightmost digit, 7, is inthe thousandths position.
The rightmost digit, 3, is inthe hundredths position.
One quick way to write a decimal as a common fraction is to remember that the numberof decimal places must be the same as the number of zeros in the denominator of the com-mon fraction.
Writing a Decimal Number as a Mixed Number
Write each decimal as a common fraction or mixed number.
The same method can be used with decimals that are greater than 1. Here the result will bea mixed number.
NOTE An informal way ofreading decimals is to simplyread the digits in order and usethe word “point” to indicatethe decimal point. 2.58 can beread “two point five eight.”0.689 can be read “zero pointsix eight nine.”
NOTE The number of digits tothe right of the decimal point iscalled the number of decimalplaces in a decimal number. So,0.35 has two decimal places.
Example 4
C H E C K Y O U R S E L F 4
Write 2.58 in words.
Example 5
When the decimal has no whole-number part, we have chosen to write a 0 to the left ofthe decimal point. This simply makes sure that you don’t miss the decimal point. However,both 0.5321 and .5321 are correct.
NOTE The 0 to the right ofthe decimal point is a“placeholder” that is notneeded in the common-fractionform.
�
294 CHAPTER 4 DECIMALS
It is often useful to compare the sizes of two decimal fractions. One approach to com-paring decimals uses the following fact.
Adding zeros to the right does not change the value of a decimal. 0.53 is the same as0.530. Look at the fractional form:
The fractions are equivalent. We have multiplied the numerator and denominator by 10.Let’s see how this is used to compare decimals in our next example.
53
100�
530
1000
Comparing the Sizes of Two Decimal Numbers
Which is larger?
0.84 or 0.842
Write 0.84 as 0.840. Then we see that 0.842 (or 842 thousandths) is greater than 0.840(or 840 thousandths), and we can write
Complete the statement below, using the symbol � or �.
0.588 ______ 0.59
REMEMBER: By theFundamental Principle ofFractions, multiplying thenumerator and denominator ofa fraction by the same nonzeronumber does not change thevalue of the fraction.
Whenever a decimal represents a measurement made by some instrument (a rule or ascale), the decimals are not exact. They are accurate only to a certain number of places andare called approximate numbers. Usually, we want to make all decimals in a particularproblem accurate to a specified decimal place or tolerance. This will require rounding thedecimals. We can picture the process on a number line.
Rounding to the Nearest Tenth
3.74 is rounded down to the nearest tenth, 3.7. 3.78 is rounded up to 3.8.
3.7
3.74
3.78
3.8
Example 7
NOTE 3.74 is closer to 3.7 thanit is to 3.8. 3.78 is closer to 3.8.
PLACE VALUE AND ROUNDING SECTION 4.1 295
Rather than using the number line, the following rule can be applied.
47. 6.734 two decimal places 48. 12.5467 three decimal places
49. 6.58739 four decimal places 50. 503.824 two decimal places
Round 56.35829 to the nearest:
51. Tenth 52. Ten thousandth
53. Thousandth 54. Hundredth
In exercises 55 to 60, determine the decimal that corresponds to the shaded portion ofeach “decimal square.” Note that the total value of a decimal square is 1.
68. Plot the following on a number line: 5.73 and 5.74. Then estimate the location for5.782.
69. Determine the reading of the Fahrenheit thermometer shown.
70. Determine the length of the pencil shown in the figure.
71. (a) What is the difference in the value of the following: 0.120, 0.1200 and0.12000?
(b) Explain in your own words why placing zeros to the right of a decimal point doesnot change the value of the number.
72. Lula wants to round 76.24491 to the nearest hundredth. She first rounds 76.24491 to76.245 and then rounds 76.245 to 76.25 and claims that this is the final answer. Whatis wrong with this approach?